Glass-based soi structures

ABSTRACT

Semiconductor-on-insulator (SOI) structures, including large area SOI structures, are provided which have one or more regions composed of a layer ( 15 ) of a substantially single-crystal semiconductor (e.g., doped silicon) attached to a support substrate ( 20 ) composed of an oxide glass or an oxide glass-ceramic. The oxide glass or oxide glass-ceramic is preferably transparent and preferably has a strain point of less than 1000° C., a resistivity at 250° C. that is less than or equal to 10 16  Ω-cm, and contains positive ions (e.g., alkali or alkaline-earth ions) which can move within the glass or glass-ceramic in response to an electric field at elevated temperatures (e.g., 300-1000° C.). The bond strength between the semiconductor layer ( 15 ) and the support substrate ( 20 ) is preferably at least 8 joules/meter 2 . The semiconductor layer ( 15 ) can include a hybrid region ( 16 ) in which the semiconductor material has reacted with oxygen ions originating from the glass or glass-ceramic. The support substrate ( 20 ) preferably includes a depletion region ( 23 ) which has a reduced concentration of the mobile positive ions.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 USC §119(e) of U.S.Provisional Application No. 60/448,176 filed Feb. 18, 2003, the contentsof which in its entirety is hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to semiconductor-on-insulator (SOI) structures.More particularly, the invention relates to 1) methods for making suchstructures and 2) novel forms of such structures.

To date, the semiconductor material most commonly used insemiconductor-on-insulator structures has been silicon. Such structureshave been referred to in the literature as silicon-on-insulatorstructures and the abbreviation “SOI” has been applied to suchstructures. The present invention relates to semiconductor-on-insulatorstructures in general, including silicon-on-insulator structures.

For ease of presentation, the following discussion will at times be interms of silicon-on-insulator structures. The references to thisparticular type of semiconductor-on-insulator structure are made tofacilitate the explanation of the invention and are not intended to, andshould not be interpreted as, limiting the invention's scope in any way.

The SOI abbreviation is used herein to refer tosemiconductor-on-insulator structures in general, including, but notlimited to, silicon-on-insulator structures. Similarly, the SOGabbreviation is used to refer to semiconductor-on-glass structures ingeneral, including, but not limited to, silicon-on-glass structures. TheSOG nomenclature is also intended to includesemiconductor-on-glass-ceramic structures, including, but not limitedto, silicon-on-glass-ceramic structures. The abbreviation SOIencompasses SOGs.

BACKGROUND OF THE INVENTION

Silicon-on-insulator technology is becoming increasingly important forhigh performance thin film transistors, solar cells, and displays, suchas, active matrix displays. The silicon-on-insulator wafers consist of athin layer of substantially single crystal silicon (generally 0.1-0.3microns in thickness but, in some cases, as thick as 5 microns) on aninsulating material.

Various ways of obtaining such a wafer include epitaxial growth of Si onlattice matched substrates; bonding of a single crystal silicon wafer toanother silicon wafer on which an oxide layer of SiO₂ has been grown,followed by polishing or etching of the top wafer down to, for example,a 0.1 to 0.3 micron layer of single crystal silicon; or ion-implantationmethods in which either hydrogen or oxygen ions are implanted either toform a buried oxide layer in the silicon wafer topped by Si in the caseof oxygen ion implantation or to separate (exfoliate) a thin Si layer tobond to another Si wafer with an oxide layer as in the case of hydrogenion implantation. Of these three approaches, the approaches based on ionimplantation have been found to be more practical commercially. Inparticular, the hydrogen ion implantation method has an advantage overthe oxygen implantation process in that the implantation energiesrequired are less than 50% of that of oxygen ion implants and the dosagerequired is two orders of magnitude lower.

Exfoliation by the hydrogen ion implantation method was initially taughtin, for example, Bister et al., “Ranges of the 0.3-2 MeV H⁺ and 0.7-2MeV H₂ ⁺ Ions in Si and Ge,” Radiation Effects, 1982, 59:199-202, andhas been further demonstrated by Michel Bruel. See Bruel, U.S. Pat. No.5,374,564; M. Bruel, Electronic Lett. 31, 1995 pp 1201-1202; and L.Dicioccio, Y. Letiec, F. Letertre, C. Jaussad and M. Bruel, ElectronicLett. 32, 1996, pp 1144-1145.

The method typically consists of the following steps. A thermal oxidelayer is grown on a single crystal silicon wafer. Hydrogen ions are thenimplanted into this wafer to generate subsurface flaws. The implantationenergy determines the depth at which the flaws are generated and thedosage determines flaw density. This wafer is then placed into contactwith another silicon wafer (the support substrate) at room temperatureto form a tentative bond.

The wafers are then heat-treated to about 600° C. to cause growth of thesubsurface flaws for use in separating a thin layer of silicon from theSi wafer. The resulting assembly is then heated to a temperature above1,000° C. to fully bond the Si film with SiO₂ underlayer to the supportsubstrate, i.e., the unimplanted Si wafer. This process thus forms asilicon-on-insulator structure with a thin film of silicon bonded toanother silicon wafer with an oxide insulator layer in between.

Cost is an important consideration for commercial applications of SOIstructures. To date, a major part of the cost of such structures hasbeen the cost of the silicon wafer which supports the oxide layer,topped by the Si thin film, i.e., a major part of the cost has been thesupport substrate.

Although the use of quartz as a support substrate has been mentioned invarious patents (see U.S. Pat. Nos. 6,140,209 6,211,041, 6,309,950,6,323,108, 6,335,231, and 6,391,740), quartz is itself a relativelyexpensive material. In discussing support substrates, some of the abovereferences have mentioned quartz glass, glass, and glass-ceramics. Othersupport substrate materials listed in these references include diamond,sapphire, silicon carbide, silicon nitride, ceramics, metals, andplastics.

As the present inventors discovered, it is not at all a simple matter toreplace a silicon wafer with a wafer made out of a less expensivematerial in an SOI structure. In particular, it is difficult to replacea silicon wafer with a glass or glass-ceramic of the type which can bemanufactured in large quantities at low cost, i.e., it is difficult tomake cost effective SOG structures. This is so because prior to thepresent invention, the art has not had practical techniques for usingglass or glass-ceramics as support substrates insemiconductor-on-insulator structures.

The present invention provides such techniques and thus satisfies thelongstanding need in the art for lower cost substrates for SOIstructures. In addition, the invention provides novel forms for suchstructures. Among the numerous applications for the invention are thosein such fields as optoelectronics, RF electronics, and mixed signal(analog/digital) electronics, as well as display applications, e.g.,LCDs and OLEDs, where significantly enhanced performance can be achievedcompared to amorphous and polysilicon based devices. In addition,photovoltaics and solar cells with high efficiency are also enabled.Both the invention's novel processing techniques and its novel SOIstructures significantly lower the cost of an SOI structure and thussatisfy the continuing demand in the semiconductor field for lower costdevices.

SUMMARY OF THE INVENTION

In accordance with a first aspect, the invention provides a method forproducing a semiconductor-on-insulator structure, specifically, an SOGstructure, comprising:

-   -   (A) providing first and second substrates (10,20) wherein:        -   (1) the first substrate (10) comprises a first external            surface (11) for bonding to the second substrate (the first            bonding surface), a second external surface (12) for            applying force to the first substrate (the first            force-applying surface), and an internal zone (13) for            separating the first substrate into a first part (14) and a            second part (15) (the internal zone (13) is hereinafter            referred to as the “separation zone,” which, for example,            can be a hydrogen ion implantation zone), wherein:            -   (a) the first bonding surface (11), the first                force-applying surface (12), and the separation zone                (13) are substantially parallel to one another;            -   (b) the second part (15) is between the separation zone                (13) and the first bonding surface (11); and            -   (c) the first substrate (10) comprises a substantially                single-crystal semiconductor material; and        -   (2) the second substrate (20) comprises two external            surfaces (21,22), one for bonding to the first substrate            (the second bonding surface) and another for applying force            to the second substrate (the second force-applying surface),            wherein:            -   (a) the second bonding surface (21) and the second                force-applying surface (22) are substantially parallel                to one another and are separated from one another by a                distance D₂; and            -   (b) the second substrate (20) comprises an oxide glass                or an oxide glass-ceramic;    -   (B) bringing the first and second bonding surfaces (11,21) into        contact (once brought into contact, the first and second bonding        surfaces form what is referred to herein as the “interface”        between the first and second substrates);    -   (C) for a period of time sufficient for the first and second        substrates to bond to one another at the first and second        bonding surfaces (i.e., at the interface), simultaneously:        -   (1) applying force to the first and second force-applying            surfaces (12,22) to press the first and second bonding            surfaces (11,21) together;        -   (2) subjecting the first and second substrates (10,20) to an            electric field which is characterized by first and second            voltages V₁ and V₂ at the first and second force-applying            surfaces (12,22), respectively, said voltages being uniform            at those surfaces with V₁ being higher than V₂ so that the            electric field is directed from the first substrate (10) to            the second substrate (20); and        -   (3) heating the first and second substrates (10,20), said            heating being characterized by first and second temperatures            T₁ and T₂ at the first and second force-applying surfaces            (12,22), respectively, said temperatures being uniform at            those surfaces and being selected so that upon cooling to a            common temperature, the first and second substrates (10,20)            undergo differential contraction to thereby weaken the first            substrate (10) at the separation zone (13); and    -   (D) cooling the bonded first and second substrates (10,20)        (e.g., to a common temperature such as room temperature) and        separating the first and second parts (14,15) at the separation        zone (13);        wherein the oxide glass or oxide glass-ceramic has one or both        of the following sets of characteristics:

(i) the oxide glass or oxide glass-ceramic has a strain point of lessthan 1,000° C. and comprises positive ions (e.g., alkali oralkaline-earth ions) which during step (C), move within the secondsubstrate (20) in a direction away from the second bonding surface (21)and towards the second force-applying surface (22); and/or

(ii) the oxide glass or oxide glass-ceramic comprises (a) non-bridgingoxygens and (b) positive ions (e.g., alkali or alkaline-earth ions)which during step (C), move within the second substrate (20) in adirection away from the second bonding surface (21) and towards thesecond force-applying surface (22).

As known in the art, non-bridging oxygens in an oxide glass or in theglass phase of an oxide glass-ceramic are those oxygens contributed tothe glass by non-network forming components of the glass. For example,in the case of commercially available LCD display glass such as CORNINGINCORPORATED GLASS COMPOSITION NO. 1737 and CORNING INCORPORATED GLASSCOMPOSITION NO. EAGLE 2000™, the non-bridging oxygens include thoseoxygens which are part of the glass through the incorporation ofalkaline-earth oxides (e.g., MgO, CaO, SrO, and/or BaO) in the glasscomposition.

Although not wishing to be bound by any particular theory of operation,it is believed that an electrolysis-type reaction takes place duringstep (C). In particular, it is believed that the semiconductor substrate(first substrate) serves as the positive electrode for theelectrolysis-type reaction and that reactive oxygen is produced in theregion of the interface between the first and second substrates. Thisoxygen is believed to react with the semiconductor material (e.g.,silicon) forming, in situ, a hybrid region (16) of oxidizedsemiconductor (e.g., a silicon oxide region for a silicon-basedsemiconductor). This hybrid region begins at the interface and extendsinto the first substrate. The presence of non-bridging oxygens in theoxide glass or oxide glass-ceramic of the second substrate is believedto play a role in the generation of the oxygens that react with thesemiconductor material of the first substrate.

It is believed that such generation of reactive oxygen and itscombination with the semiconductor material is a source of the strongbond which the invention achieves between the semiconductor material ofthe first substrate and the oxide glass or oxide glass-ceramic of thesecond substrate, i.e., at least a part (and potentially all) of thebond between the first and second substrates is through the reaction ofthe semiconductor material with reactive oxygen originating from thesecond substrate. Significantly, unlike prior techniques, this strongbond is achieved without the need for a high temperature treatment,i.e., a treatment at a temperature above 1,000° C.

This ability to avoid high temperature processing allows the secondsubstrate to be a material which can be manufactured in large quantitiesat low cost. That is, by eliminating high temperature processing, theinvention eliminates the need for a support substrate composed of anexpensive high temperature material, such as, silicon, quartz, diamond,sapphire, etc.

In particular, the ability to achieve a strong bond without the need fora high temperature treatment allows the second substrate to be composedof an oxide glass or an oxide glass-ceramic; in one embodiment the glassor glass-ceramic exhibits a strain point less than 1,000° C. Moreparticularly, for display applications, the oxide glass or oxideglass-ceramic typically has a strain point less than 800° C., and infurther embodiments less than 700° C. For electronics and otherapplications, the strain point is preferably less than 1,000° C. As wellknown in the glass making art, glasses and glass-ceramics having lowerstrain points are easier to manufacture than glasses and glass-ceramicshaving higher strain points.

To facilitate bonding, the oxide glass or oxide glass-ceramic should beable to conduct electricity at least to some extent. The conductivity ofoxide glasses and oxide glass-ceramics depends on their temperature andthus in achieving a strong bond between the semiconductor material andthe oxide glass or oxide glass-ceramic, there is a balance among: 1) theconductivity of the glass or glass-ceramic, 2) the temperatures (T₁ andT₂) used in step (C), 3) the strength of the electric field applied tothe first and second substrates during step (C), and 4) the amount oftime during which step (C) is performed.

As a general guideline, the oxide glass or oxide glass-ceramicpreferably has a resistivity p at 250° C. that is less than or equal to10¹⁶ Ω-cm (i.e., a conductivity at 250° C. that is greater than or equalto 10¹⁶ Siemens/cm). More preferably, ρ at 250° C. is less than or equalto 10¹³ Ω-cm, and most preferably, it is less than or equal to 10^(11.5)Ω-cm. It should be noted that although quartz has the requisiteresistivity at 250° C. of 10^(11.8) Ω-cm, it lacks positive ions thatcan move during step (C), and it thus follows that quartz is unsuitablefor use as the second substrate in producing SOI structures inaccordance with the above procedures.

For any particular set of first and second substrates, persons skilledin the art will readily be able to determine suitable combinations oftime, temperature, and field strength for step (C) from the presentdisclosure. In particular, such persons will be able to selectcombinations of these parameters which create a bond between thesemiconductor and the oxide glass or oxide glass-ceramic which is strongenough for the SOI structure to withstand the various forces andenvironmental conditions to which it will be exposed during furtherprocessing and/or use.

In addition to the above role in bonding, the electric field applied instep (C) also moves positive ions (cations) within the second substratein a direction from the second substrate's bonding surface (the secondbonding surface) towards its force-applying surface (the secondforce-applying surface). Such movement preferably forms a depletionregion (23) which begins at the interface between the first and secondsubstrates and extends into the second substrate, i.e., the depletionregion begins at the second bonding surface and extends into the secondsubstrate towards the second force-applying surface.

The formation of such a depletion region is especially desirable whenthe oxide glass or oxide glass-ceramic contains alkali ions, e.g., Li⁺¹,Na⁺¹, and/or K⁺¹ ions, since such ions are known to interfere with theoperation of semiconductor devices. Alkaline-earth ions, e.g., Mg⁺²,Ca⁺², Sr⁺², and/or Ba⁺², can also interfere with the operation ofsemiconductor devices and thus the depletion region also preferably hasreduced concentrations of these ions.

Significantly, it has been found that the depletion region once formedis stable over time even if the SOI structure is heated to an elevatedtemperature comparable to, or even to some extent higher than, that usedin step (C). Having been formed at an elevated temperature, thedepletion region is especially stable at the normal operating andformation temperatures of SOI structures. These considerations ensurethat alkali and alkaline-earth ions will not diffuse back from the oxideglass or oxide glass-ceramic into the semiconductor of the SOI structureduring use or further device processing, which is an important benefitderived from using an electric field as part of the bonding process ofstep (C).

As with selecting the operating parameters to achieve a strong bond, theoperating parameters needed to achieve a depletion region of a desiredwidth and a desired reduced positive ion concentration for all of thepositive ions of concern can be readily determined by persons skilled inthe art from the present disclosure. When present, the depletion regionis a characteristic feature of an SOI structure produced in accordancewith the method aspects of the present invention.

In addition to the depletion region, the application of the electricfield can also create “pile-up” regions for one or more of the mobilepositive ions contained in the oxide glass or oxide glass-ceramic. Whenpresent, such regions are located at or near the side (edge) of thedepletion region farthest from the interface between the first andsecond substrates. Within the pile-up region, the positive ion has aconcentration above its bulk concentration. For example, when measuredin atomic percent, the peak concentration of the positive ion in thepile-up region can be, for example, up to 5 times greater than the bulkconcentration. Like the depletion region, such a pile-up region, whenpresent, is a characteristic feature of an SOI structure produced inaccordance with the method aspects of the present invention.

The temperatures of the first and second substrates during step (C),i.e., the values of T₁ and T₂, are chosen to perform the importantfunction of weakening (e.g., fracturing) the semiconductor substrate(first substrate) at the separation zone so that the first substrate canbe divided into first and second parts, the second part being bonded tothe second substrate. In this way, an SOI structure having asemiconductor portion of a desired thickness is achieved, e.g., athickness D_(S) between, for example, 10 nanometers and 500 nanometersand, in some cases, up to 5 microns.

Although not wishing to be bound by any particular theory of operation,it is believed that the weakening of the semiconductor substrate at theseparation zone primarily occurs as the bonded first and secondsubstrates are cooled after step (C), e.g., to room temperature. By theproper selection of T₁, and T₂ (see below), this cooling causes thefirst and second substrates to differentially contract. Thisdifferential contraction applies stress to the first substrate whichmanifests itself as a weakening/fracturing of the first substrate at theseparation zone. As discussed below, preferably, the differentialcontraction is such that the second substrate seeks to contract morethan the first substrate.

As used herein, the phrase “differential contraction upon cooling to acommon temperature” and similar phrases mean that if the first andsecond substrates were not bonded, they would contract to differentextents by such cooling. However, since the first and second substratesbecome bonded during step (C) and are rigid materials, the amount ofcontraction of the individual substrates which actually occurs will bedifferent from that which would occur if there were no bonding. Thisdifference leads to one of the substrates experiencing tension and theother compression as a result of the cooling. The phrase “seeks tocontract” and similar phrases are used herein to reflect the fact thatthe contraction of the substrates when bonded will in general bedifferent from their non-bonded contraction, e.g., the substrate beingdiscussed may seek to contract to a certain extent as a result of thecooling but may not and, in general, will not actually contract to thatextent as a result of being bonded to the other substrate.

The values of T₁ and T₂ used during step (C) will depend on the relativecoefficients of thermal expansion of the first and second substrates,the goal in choosing these values being to ensure that one of thesubstrates, preferably, the second substrate, seeks to contract to agreater extent than the other substrate, preferably, the firstsubstrate, so as to apply stress to, and thus weaken, the separationzone during cooling.

In general terms, in order for the second substrate to seek to contractto a greater extent than the first substrate during cooling, T₁, T₂, andthe CTE's of the first and second substrates (CTE₁, and CTE₂,respectively) should satisfy the relationship:

CTE₂ ·T ₂>CTE₁ ·T ₁,

where CTE₁ is the 0° C. coefficient of thermal expansion of thesubstantially single-crystal semiconductor material and CTE₂ is the0-300° C. coefficient of thermal expansion of the oxide glass or oxideglass-ceramic. This relationship assumes that the first and secondsubstrates are cooled to a common reference temperature of 0° C.

In applying this relationship, it should be kept in mind that the oxideglass or oxide glass-ceramic preferably has a 0-300° C. coefficient ofthermal expansion CTE which satisfies the relationship:

5×10⁻⁷/° C.≦CTE≦75×10⁻⁷/° C.

For comparison, the 0° C. coefficient of thermal expansion ofsubstantially single-crystal silicon is approximately 24×10⁻⁷/° C.,while the 0-300° C. average CTE is approximately 32.3×10⁻⁷/° C. Althougha CTE for the second substrate which is less than or equal to 75×10⁻⁷/°C. is generally preferred, in some cases, the CTE of the secondsubstrate can be above 75×10⁻⁷/° C., e.g., in the case of soda limeglasses for use in such applications as solar cells.

As can be seen from the CTE₂·T₂>CTE₁·T₂ relationship, when the CTE ofthe oxide glass or oxide glass-ceramic (CTE₂) is less than that of thesemiconductor material (CTE₁), a larger T₂−T₁ difference will be neededin order for the second substrate to seek to contract more than thefirst substrate during cooling. Conversely, if the CTE of the oxideglass or oxide glass-ceramic is greater than that of the semiconductormaterial, a smaller T₂−T₁ difference can be used. Indeed, if the CTE ofthe oxide glass or oxide glass-ceramic is sufficiently above than thatof the semiconductor material, the T₂−T₁ difference can become zero oreven negative. However, in general, the CTE of the oxide glass or oxideglass-ceramic is chosen to be relatively close to that of thesemiconductor material so that a positive T₂−T₁ difference is needed toensure that the second substrate will seek to contract more than thefirst substrate during cooling. Having T₂>T₁ is also desirable since itcan aid in bonding of the oxide glass or oxide glass-ceramic to thesemiconductor material since it tends to make the oxide glass or oxideglass-ceramic more reactive. Also, having T₂>T₁ is desirable since itcan facilitate movement of positive ions away from the interface betweenthe first and second substrates.

The differential contraction between the first and second substratesduring cooling and the resulting weakening/fracturing of the firstsubstrate at the separation zone can be achieved by approaches otherthan having the second substrate seek to contract more than the firstsubstrate during the cooling. In particular, it can be the firstsubstrate that seeks to contract more than the second substrate. Again,this differential contraction is achieved through the selection of theCTE's and temperatures of the first and second substrates. In generalterms, for this case, CTE₁·T₁ needs to be greater than CTE₂·T₂.

When the first substrate seeks to contract more than the secondsubstrate, the first substrate and, in particular, the second part ofthe first substrate, will end up under tension, rather than undercompression, at the end of the cooling. In general, it is preferred forthe semiconductor film (second part of the first substrate) to be undercompression in the finished SOI structure, which makes the approach inwhich the differential contraction causes the second substrate to seekto contract more than the first substrate during cooling preferred. Forsome applications, however, having the semiconductor film under sometension may be preferred.

For example, having the semiconductor film under some tension may beuseful in obtaining higher electron mobilities through the “strainedsilicon effect.” See, for example, U.S. Pat. Nos. 5,442,205, 6,107,653,6,573,126, and 6,593,641. Such a condition for the semiconductor filmcan be readily achieved in accordance with the present invention throughthe choice of T₁, T₂, and CTE₂ for a given CTE₁. Examples of oxideglasses and oxide-glass ceramics which have relatively low CTE's andthus will contract less than a semiconductor film, e.g., a siliconsemiconductor film, include those oxide glasses and glass-ceramicsexhibiting a CTE of less than about 15×10⁻⁷/° C. (0-300° C.).

Thus, to summarize, although other sets of conditions can be used in thepractice of the invention, in the preferred embodiments of theinvention, T₂ is greater than T₁ during step (C) and the secondsubstrate seeks to contract more than the first substrate during coolingfrom the elevated temperatures used during step (C).

Again, for any particular application of the invention (e.g., anyparticular semiconductor material and any particular oxide glass oroxide glass-ceramic), persons skilled in the art will readily be able toselect values for T₁ and T₂ based on the present disclosure which willprovide a level of differential contraction sufficient to weaken theseparation zone so that the first and second parts of the firstsubstrate can be separated from one another to produce the desired SOIstructure.

As discussed in further detail below in connection with FIG. 1D,separation of the first and second parts at the separation zone resultsin each part having an“exfoliation” surface where the separationoccurred. As known in the art, upon initial formation, i.e., before anysubsequent surface treatments, such an exfoliation surface ischaracterized by a surface roughness which is generally at least on theorder of 0.5 nanometers RMS, e.g., in the range of 1-100 nanometers, anddepending on the process conditions used, will typically have aconcentration of the implanted ion used to form the separation zone,e.g., hydrogen, above that present in the body of the first or secondparts. The exfoliation surface as initially formed will also becharacterized by a distorted crystal structure as seen by TEM. Intypical applications, the exfoliation surface is polished prior to useso that its RMS surface roughness is reduced to 1 nanometer or less,e.g., to a RMS surface roughness on the order of 0.1 nanometers forelectronic applications. As used herein, the phrase “exfoliationsurface” includes the surface as initially formed and the surface afterany subsequent treatments.

The pressure applied to the first and second substrates during step (C)ensures that those substrates are in intimate contact while undergoingthe heat and electric field treatments of that step. In this way, strongbonding between the substrates can be achieved.

Generally, the semiconductor substrate (the first substrate) will beable to withstand higher levels of applied pressure than the glass orglass-ceramic substrate (the second substrate). Thus, the pressure ischosen to provide intimate contact between the substrates withoutdamaging the second substrate.

A wide range of pressures can be used. For example, the force per unitarea P applied to the first and second force-applying surfaces of thefirst and second substrates, respectively, preferably satisfies therelationship:

1 psi≦P≦100 psi;

and most preferably, the relationship:

1 psi≦P≦50 psi.

Again, the specific pressure value to be used for any particularapplication of the invention can be readily determined by personsskilled in the art from the present disclosure.

The first aspect of the invention can be practiced using a single firstsubstrate and a single second substrate. Alternatively, the methods ofthe invention can be used to form more than one SOI structure on asingle second substrate.

For example, steps (A) through (D) can be used to form a first SOIstructure which does not cover the entire area of the second substrate.Thereafter, steps (A) through (D)) can be repeated to form a second SOIstructure which covers all or part of the area not covered by the firstSOI structure. The second SOI structure may be the same or differentfrom the first SOI structure, e.g., the second SOI structure can be madeusing a first substrate composed of a substantially single-crystalsemiconductor material that is the same or different from thesemiconductor material of the first substrate used in producing thefirst SOI structure.

More preferably, multiple SOI structures are formed simultaneously on asingle second substrate by providing multiple (i.e., two or more) firstsubstrates in step (A), bringing all of those first substrates intocontact with a single second substrate in step (B), and then performingsteps (C) and (D) on the resulting multiple first substrate/singlesecond substrate assembly. The multiple first substrates provided instep (A) can all be the same, all different, or some the same and somedifferent.

Whichever approach is used, the resulting multiple SOI structures on asingle oxide glass or oxide glass-ceramic substrate can be contiguous orseparated as appropriate for the particular application of theinvention. If desired, gaps between some or all of the adjacentstructures can be filled with, for example, semiconductor material toobtain one or more continuous semiconductor layers on an oxide glass oroxide glass-ceramic substrate of any desired size.

In addition to the foregoing method aspects, the invention also providesnovel SOI structures.

Thus, in accordance with a second aspect, the invention provides asemiconductor-on-insulator structure comprising first and second layers(15,20) which are attached to one another either directly or through oneor more intermediate layers, wherein:

(a) the first layer (15) comprises a substantially single-crystalsemiconductor material;

(b) the second layer (20) comprises an oxide glass or an oxideglass-ceramic; and

(c) the bond strength between the first and second layers is at least 8joules/meter², preferably at least 10 joules/meter², and most preferablyat least 15 joules/meter².

As used throughout this specification and in the claims, the bondstrength between a semiconductor layer and a glass or glass-ceramiclayer of an SOI structure is determined using an indentation procedure.Such procedures are widely used to assess the adhesion characteristicsof tin films and coatings to a wide variety of materials, includingpolymeric, metallic, and brittle materials. The technique provides aquantitative measure of adhesion in the form of the interfacial strainenergy release rate.

In the examples presented below, indentation measurements of siliconcoatings on glass were performed using a Nano Indenter II (MTS SystemsCorporation, Eden Prairie, Minn.) equipped with a Berkovich diamondindenter. Other equipment can, of course, be used to determine bondstrength values. As discussed in detail in Example 12 below,indentations were made covering a range of loads and the regionimmediately surrounding the indentations was examined for evidence ofdelamination. Calculations of bond energies were made in accordance withthe following reference, the relevant portions of which are incorporatedherein by reference: D. B. Marshall and A. G. Evans, Measurement ofAdherence of Residually Stressed Thin Films by Indentation. I. Mechanicsof Interface Delamination, J. Appl. Phys., 56 [10] 2632-2638 (1984). Theprocedures of this reference are to be used in calculating the bondenergies called for by the claims set forth below.

When the SOI structure is produced using, for example, the first aspectof the invention, the first layer will have a surface (second face 13 b)farthest from the second layer which is an exfoliation surface. In thiscase, the oxide glass or oxide glass ceramic of the second layer willalso preferably have:

-   -   (a) a 0-300° C. coefficient of thermal expansion CTE and a        250° C. resistivity ρ which satisfy the relationships:

5×10⁻⁷/° C.≦CTE≦75×10⁻⁷/° C., and

ρ≦10¹⁶ Ω-cm, and

-   -   (b) a strain point T_(S) of less than 1,000° C.        The oxide glass or oxide glass ceramic will also comprise        positive ions whose distribution within the oxide glass or oxide        glass-ceramic can be altered by an electric field when the        temperature T of the oxide glass or oxide glass-ceramic        satisfies the relationship:

T _(S)−350≦T≦T _(S)+350,

where T_(S) and T are in degrees centigrade.

As will be appreciated, the strength of the bond between the glass orglass-ceramic layer and the semiconductor layer, e.g., silicon layer,attached thereto is a key property of an SOI structure. High bondstrength and durability are very important to ensure that the SOIstructure can withstand the processing associated with the manufactureof thin film transistors and other devices on or within the structure.For example, a high bond strength is important in providing deviceintegrity during cutting, polishing, and similar processing steps. Ahigh bond strength also allows semiconductor films of variousthicknesses to be processed while attached to glass or glass-ceramicsubstrates, including thin semiconductor films.

It is known that the bond energy for the Si—SiO₂ bond for the standardthermal process for producing SOI structures depends on the annealingtemperature and is in the range of 1-4 joules/meter after a 1100° C.anneal. See Semiconductor Wafer Bonding, Q. Y. Tong, U. Gosele, JohnWiley & Sons Inc., New York, N.Y., page 108, (1994). As demonstrated bythe examples set forth below, in accordance with the second aspect ofthe invention, bond strengths for SOI structures much higher than thosepreviously achieved are provided, i.e., bond strengths of at least 8joules/meter².

In accordance with a third aspect, the invention provides asemiconductor-on-insulator structure comprising first and second layers(15,20) which are attached to one another either directly or through oneor more intermediate layers, wherein:

(a) the first layer (15):

-   -   (i) comprises a substantially single-crystal semiconductor        material;    -   (ii) has first and second substantially parallel faces (11,13 b)        separated by a distance Ds, the first face (11) being closer to        the second layer (20) than the second face (13 b);    -   (iii) has a reference surface (17) which 1) is within the first        layer (15), 2) is substantially parallel to the first face (11),        and 3) is separated from that face by a distance D_(S)/2; and    -   (iv) has a region (16) of enhanced oxygen concentration which        begins at the first face (11) and extends towards the second        face (13 b), said region having a thickness δ_(H) which        satisfies the relationship:

δ_(H)≦200 nanometers,

where δ_(H) is the distance between the first face (11) and a surface(16 a) which 1) is within the first layer (15), 2) is substantiallyparallel to the first face (11), and 3) is the surface farthest from thefirst face (11) for which the following relationship is satisfied:

C_(O)(x)−C_(O/Ref)≦50 percent, 0≦x≦δ _(H),

where:

C_(O)(x) is the concentration of oxygen as a function of distance x fromthe first face (11),

C_(O/Ref) is the concentration of oxygen at the reference surface (17),and

C_(O)(x) and C_(O/Ref) are in atomic percent; and

(b) the second layer (20) comprises an oxide glass or an oxideglass-ceramic.

It should be noted that the region of enhanced oxygen concentration ofthis aspect of the invention is to be distinguished from an oxide layerformed on the outside of the semiconductor substrate prior to bonding(see, for example, U.S. Pat. No. 5,909,627) in that the region of thepresent invention is within the semiconductor material. In particular,when the SOI structure is produced using, for example, the first aspectof the invention, the region of enhanced oxygen concentration is formedin situ as the composite of the semiconductor layer and the oxide glassor oxide glass-layer is formed.

In accordance with a fourth aspect, the invention provides asemiconductor-on-insulator structure comprising first and second layers(15,20) which are attached to one another either directly or through oneor more intermediate layers, wherein:

(a) the first layer (15) comprises a substantially single-crystalsemiconductor material, said layer having a surface (second face 13 b)farthest from the second layer which is an exfoliation surface; and

(b) the second layer (20):

-   -   (i) has first and second substantially parallel faces (21,22)        separated by a distance D₂, the first face (21) being closer to        the first layer (15) than the second face (22);    -   (ii) has a reference surface (24) which 1) is within the second        layer (20), 2) is substantially parallel to the first face (21),        and 3) is separated from that face by a distance D₂/2;    -   (iii) comprises an oxide glass or an oxide glass-ceramic which        comprises positive ions of one or more types, each type of        positive ion having a reference concentration C_(i/Ref) at the        reference surface; and    -   (iv) has a region (23) which begins at the first face (21) and        extends towards the reference surface (24) in which the        concentration of at least one type of positive ion is depleted        relative to the reference concentration C_(i/Ref) for that ion        (the positive ion depletion region).

In accordance with a fifth aspect, the invention provides asemiconductor-on-insulator structure comprising first and second layers(15,20) which are attached to one another either directly or through oneor more intermediate layers, wherein:

(a) the first layer (15) comprises a substantially single-crystalsemiconductor material, said layer having a thickness of less than 10microns (preferably, less than 5 microns, more preferably, less than 1micron); and

(b) the second layer (20):

-   -   (i) has first and second substantially parallel faces (21,22)        separated by a distance D₂, the first face (21) being closer to        the first layer (15) than the second face (22);    -   (ii) has a reference surface (24) which 1) is within the second        layer (20), 2) is substantially parallel to the first face (21),        and 3) is separated from that face by a distance D₂/2;    -   (iii) comprises an oxide glass or an oxide glass-ceramic which        comprises positive ions of one or more types, each type of        positive ion having a reference concentration C_(i/Ref) at the        reference surface; and    -   (iv) has a region (23) which begins at the first face (21) and        extends towards the reference surface (24) in which the        concentration of at least one type of positive ion is depleted        relative to the reference concentration C_(i/Ref) for that ion        (the positive ion depletion region).

In connection with this aspect of the invention, it should be noted thatthe 10 micron limitation of subparagraph (a) is substantially less thanthe thickness of a semiconductor wafer. For example, commerciallyavailable silicon wafers have thicknesses greater than 100 microns.

In accordance with a sixth aspect, the invention provides asemiconductor-on-insulator structure comprising first and second layers(15,20) which are attached to one another either directly or through oneor more intermediate layers, wherein:

(a) the first layer (15) comprises a substantially single-crystalsemiconductor material; and

(b) the second layer (20) comprises an oxide glass or an oxideglass-ceramic which comprises positive ions of one or more types,wherein the sum of the concentrations of lithium, sodium, and potassiumions in the oxide glass or oxide glass-ceramic on an oxide basis is lessthan 1.0 weight percent and, preferably, less than 0.1 weight percent(i.e., wt. % Li₂O+wt. % K₂O+wt. % Na₂O<1.0 wt. %, preferably, <0.1 wt.%),

wherein the first layer (15) has a maximum dimension (e.g., diameter inthe case of a circular layer, diagonal in the case of a rectangularlayer, etc.) greater than 10 centimeters.

In accordance with a seventh aspect, the invention provides asemiconductor-on-insulator structure comprising first and second layers(15,20) which are attached to one another either directly or through oneor more intermediate layers, wherein:

(a) the first layer (1S) comprises a substantially single-crystalsemiconductor material; and

(b) the second layer (20):

-   -   (i) has first and second substantially parallel faces (21,22)        separated by a distance D₂, the first face (21) being closer to        the first layer (15) than the second face (22);    -   (ii) has a reference surface (24) which 1) is within the second        layer (20), 2) is substantially parallel to the first face (21),        and 3) is separated from that face by a distance D₂/2;    -   (iii) comprises an oxide glass or an oxide glass-ceramic which        comprises positive ions of one or more types, each type of        positive ion having a reference concentration C_(i/Ref) at the        reference surface;    -   (iv) has a region (23) which begins at the first face (21) and        extends towards the reference surface (24) in which the        concentration of at least one type of positive ion is depleted        relative to the reference concentration C_(i/Ref) for that ion        (the positive ion depletion region), said region having a distal        edge (23 a) (i.e., the edge closest to the reference surface);        and    -   (v) has a region (25) in the vicinity of said distal edge of the        positive ion depletion region (23) in which the concentration of        at least one type of positive ion is enhanced relative to        C_(i/Ref) for that ion (the pile-up region).

In accordance with an eighth aspect, the invention provides asemiconductor-on-insulator structure comprising first and second layers(15,20) which are attached to one another either directly or through oneor more intermediate layers with a bond strength of at least 8joules/meter², preferably at least 10 joules/meter², and most preferablyat least 15 joules/meter, said first layer (15) comprising asubstantially single-crystal semiconductor material and said secondlayer (20) comprising an oxide glass or an oxide glass-ceramic whereinat least a portion of the first layer (15) proximal to the second layer(20) comprises recesses (18) which divide said portion intosubstantially isolated regions (19) which can expand and contractrelatively independently of one another.

In certain preferred embodiments of this aspect of the invention, therecesses (18) extend through the entire thickness (Ds) of the firstlayer (15).

In accordance with a ninth aspect, the invention provides asilicon-on-insulator structure comprising first and second layers(15,20) which are directly attached to one another, said first layer(15) comprising a substantially single-crystal silicon material and saidsecond layer (20) comprising a glass or a glass-ceramic which comprisessilica and one or more other oxides as network formers (e.g., B₂O₃,Al₂O₃, and/or P₂O₅), said first layer (15) comprising a region (16)which contacts the second layer (20) and comprises silicon oxide (i.e.SiO_(X) where 1≦x≦2) but does not comprise the one or more other oxides,said region having a thickness which is less than or equal to 200nanometers.

In accordance with a tenth aspect, the invention provides asemiconductor-on-insulator structure comprising a substantiallysingle-crystal semiconductor material (material S) and an oxide glass oran oxide glass-ceramic which comprises positive ions (material G),wherein at least a part of the structure comprises in order:

-   -   material S;    -   material S with an enhanced oxygen content;    -   material G with a reduced positive ion concentration for at        least one type of positive ion;    -   material G with an enhanced positive ion concentration for at        least one type of positive ion; and    -   material G.

In connection with each of the foregoing aspects of the invention, itshould be noted that the “insulator” component of asemiconductor-on-insulator structure is automatically provided by theinvention through the use of an oxide glass or an oxide glass-ceramic asthe second substrate. The insulating function of the glass orglass-ceramic is even further enhanced when the interface (30) betweenthe first and second substrates (10,20) includes a positive iondepletion region (23). As a specific example, in the tenth aspect, allof the G materials are insulators. In addition, the S material withenhanced oxygen concentration may, at least to some extent, function asan insulator depending on the oxygen concentration achieved. In suchcases, everything after the S material constitutes the insulator of theSOI structure.

This automatic provision of the insulator function in accordance withthe invention is to be contrasted with conventional SOI structures inwhich a semiconductor film is attached to a semiconductor wafer. Toachieve an insulating function, an insulator layer, e.g., a SiO₂ layer,needs to be sandwiched (buried) between the semiconductor film and thesemiconductor wafer.

As discussed above in connection with the first aspect of the invention,the methods of the present invention can be practiced to producemultiple SOI structures on a single oxide glass or oxide glass-ceramicsubstrate, where the SOI structures may all be the same, all different,or some the same and some different. Similarly, the product aspects ofthe invention can have multiple first layers (15) on a single secondlayer (20), where again, the first layers may all be the same, alldifferent, or some the same and some different.

Whether a single first layer or a plurality of first layers are used,the resulting SOI structure can either have all or substantially all(i.e., >95%) of the first face (21) of the second layer (20) attached(either directly or through one or more intermediate layers) to one ormore kinds of substantially single-crystal semiconductor materials, orcan have substantial areas of the first face that are associated withmaterials that are not substantially single-crystal semiconductormaterials (hereinafter, the “non-single crystal semiconductor areas”).

In the non-single crystal semiconductor areas, the first face can beattached, either directly or through one or more intermediate layers,to, for example, amorphous and/or polycrystalline semiconductormaterials, e.g., amorphous and/or polycrystalline silicon. The use ofsuch less expensive materials can be particularly beneficial in displayapplications where substantially single-crystal semiconductor materialsare typically only needed for certain parts of the display electronics,e.g., for peripheral drivers, image processors, timing controllers, andthe like, that require higher performance semiconductor materials. Aswell-known in the art, polycrystalline semiconductor materials and, inparticular, polycrystalline silicon can be obtained by thermalcrystallization (e.g., laser-based thermal crystallization) of amorphousmaterials after those materials have been applied to a substrate, suchas, an LCD glass substrate.

The entire first face of the second layer, of course, does not have tobe associated with substantially single-crystal or non-single crystalsemiconductor materials. Rather, specified areas can have thesemiconductor materials with the spaces between such areas being eitherbare second layer or second layer attached to one or morenon-semiconductor materials. The sizes of such spaces can be large orsmall as appropriate to the particular application of the invention. Forexample, in the case of display applications, e.g., liquid crystaldisplays, the great majority of the glass layer (e.g., greater thanapproximately 75-80%) will typically not be associated with eithersubstantially single-crystal or non-single crystal semiconductormaterials.

Through the use of multiple first layers attached to a single secondlayer, SOI structures having extensive areas composed of substantiallysingle-crystal semiconductor materials can be obtained. Thus, inaccordance with an eleventh aspect, the invention provides asemiconductor-on-insulator structure comprising first and second layers(15,20) which are attached to one another either directly or through oneor more intermediate layers, wherein:

(a) the first layer (15) comprises a plurality of regions each of whichcomprises a substantially single-crystal semiconductor material;

(b) the second layer (20) comprises an oxide glass or an oxideglass-ceramic; and

(c) the regions have surface areas A_(i) which satisfy the relationship:

${{\sum\limits_{i = 1}^{N}A_{i}} > A_{T}},{N > 1},$

where A_(T)=750 centimeters² if any of the regions has a circularperimeter and A_(T)=500 centimeters² if none of the regions has acircular perimeter.

As above, the substantially single-crystal semiconductor materials ofthe various regions can all be the same, all different, or some the sameand some different. Similarly, if one or more intermediate layers areused, they can all be the same, all different, or some the same and somedifferent for the various regions. In particular, one or more regionscan have the substantially single-crystal semiconductor materialattached to the second layer through one or more intermediate layers,while one or more other regions can have the semiconductor materialattached directly to the second layer.

In connection with the foregoing second through eleventh aspects of theinvention, the one or more intermediate layers between the first andsecond substrates, if present, preferably have a combined thickness ofless than 100 nm, more preferably less than 50 nm, and most preferably,less than 30 nm.

In addition to the above-listed individual aspects, the invention alsocomprises any and all combinations of these aspects. For example, allembodiments of the invention preferably have an SOI structure which ischaracterized by a bond strength of at least 8 joules/meter², preferablyat least 10 joules/meter², and most preferably at least 15joules/meter². Similarly, the SOI structure preferably includes at leastone exfoliation surface, at least one positive ion depletion region, atleast one pile-up region, and/or a semiconductor layer whose thicknessis less than 10 microns.

The reference numbers used in the summaries of the various aspects ofthe invention are only for the convenience of the reader and are notintended to and should not be interpreted as limiting the scope of theinvention. More generally, it is to be understood that both theforegoing general description and the following detailed description aremerely exemplary of the invention, and are intended to provide anoverview or framework for understanding the nature and character of theinvention.

Additional features and advantages of the invention are set forth in thedetailed description which follows, and in part will be readily apparentto those skilled in the art from that description or recognized bypracticing the invention as described herein. The accompanying drawingsare included to provide a further understanding of the invention, andare incorporated in and constitute a part of this specification. Theschematic drawings are not intended to indicate scale or relativeproportions of the elements shown therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic drawing illustrating step (A) of the invention'smethod for producing SOI structures.

FIG. 1B is a schematic drawing illustrating step (B) of the invention'smethod for producing SOI structures.

FIG. 1C is a schematic drawing illustrating step (C) of the invention'smethod for producing SOI structures. As shown in this figure, T₂ ispreferably greater than T₁.

FIG. 1D is a schematic drawing illustrating step (D) of the invention'smethod for producing SOI structures.

FIG. 2A shows a completed SOI structure after step (D).

FIG. 2B is a schematic drawing illustrating at an expanded scale theinterface region 30 between the first and second substrates produced bystep (C). As indicated in this figure, reference surfaces 17 and 24 areat ½D_(S) and ½D₂ of FIG. 2A, respectively.

FIG. 3 is a schematic drawing illustrating the use of recesses in thebonding surface of the first substrate to create regions which canexpand and contract in relative isolation from one another.

FIG. 4 is a plan view of the bonding surface of the first substrateillustrating the recesses and isolated regions of FIG. 3.

FIGS. 5A and 5B are ToF-SIMS depth profiles of an SOI structure producedin accordance with the invention for a second substrate which includesboth Li⁺ and K⁺ as mobile positive ions.

FIGS. 6A and 6B are ToF-SIMS depth profiles of an SOI structure of thetype used in FIG. 5 after reheating.

FIGS. 7A and 7B are ToF-SIMS depth profiles of an SOI structure producedin accordance with the invention for a second substrate which includesessentially only Na⁺ as a mobile positive ion.

FIGS. 8A and 8B are ToF-SIMS depth profiles of an SOI structure of thetype used in FIG. 7 after reheating.

Each of FIGS. 5A, 6A, 7A, and 8A shows SIMS signal intensity data, andeach of FIGS. 5B, 6B, 7B, and 8B shows that data converted into atomicpercent values.

FIG. 9 is a convergent beam electron diffraction pattern illustratingthe single crystal nature of the semiconductor layer of an SOI structureproduced in accordance with the invention. The semiconductor film inthis case is phosphorus-doped silicon (Si).

FIGS. 10A, 10B, and 10C are ToF-SIMS depth profiles of an SOI structureproduced in accordance with the invention for a second substrate whichcomprises alkaline-earth ions as the mobile positive ions. FIGS. 10A and10B show SIMS signal intensity data, and FIG. 10C shows that dataconverted into atomic percent values.

FIG. 11 is a schematic drawing illustrating an SOI structure whichcomprises multiple first layers on a single second layer.

FIG. 12 is a schematic drawing illustrating a preferred process forproducing the SOI structure of FIG. 11.

FIGS. 13A and 13B are schematic drawings illustrating preferredapproaches for assembling multiple first substrates on a single secondsubstrate. In particular, FIG. 13A shows machining of the edges of thefirst substrates to reduce the size of the gap between the substrates,and FIG. 13B shows the assembly of the first substrates on a conductivebacker to, among other things, simplify repeated use of the firstsubstrates.

FIG. 14 is a schematic drawing illustrating a multiple firstsubstrate/single second substrate assembly where the gaps between firstsubstrates have been filled with a semiconductor material.

FIG. 15 is a schematic drawing illustrating a prior art process forforming a thin film transistor.

FIG. 16 is a schematic flow chart showing a preferred embodiment of theprocess aspects of the invention.

In the above drawings, like reference numbers designate like orcorresponding parts throughout the several views. The elements to whichthe reference numbers generally correspond are set forth in Table 1.

DETAILED DESCRIPTION OF THE INVENTION

As discussed above, a major challenge of using inexpensive materials asa support substrate and, in particular, in using glass-based materials,specifically, oxide glasses and oxide glass-ceramics, as a supportsubstrate, is that the 1100° C. bonding treatment previously used in theart cannot be used with glass-based wafers since most glasses cannotwithstand process temperatures of this magnitude. The covalent bondingbetween the glass-based layer and the semiconductor material e.g.,silicon, thus has to be achieved at significantly lower temperatures.The requirement for lower temperatures also makes it challenging toseparate the semiconductor wafer into parts at a separation zone formedby, for example, hydrogen ion implantation.

Moreover, when a glass-based material is substituted for silicon as asupport substrate material, the expansion of the glass-based materialshould be matched to the expansion of the semiconductor layer of the SOIstructure to avoid separation of the semiconductor layer from thesupport substrate. Although some glass-based materials with expansionsclose to that of semiconductors, e.g., silicon, are known, an exactmatch is difficult to obtain. Expansion mismatches are especiallytroublesome for large wafers where the stresses can become sufficientlyhigh so as to cause debonding of the semiconductor layer.

A further requirement is that the interface between the glass-basedsubstrate and the semiconductor material should be free of ions, such asalkali ions, which can diffuse into semiconductor structures and causeserious problems with the functioning of semiconductor devices. Theglass-based materials known to have reasonably matched expansions tosemiconductor materials, e.g., silicon, often contain alkali ions.

Thus, multiple problems need to be addressed and overcome to provide SOIstructures which employ support substrates composed of glass-basedmaterials. As summarized above, the present invention provides methodsfor producing SOI structures which address and overcome these problems.FIGS. 1A, 1B, 1C and 1D schematically illustrate the process.

Thus, FIG. 1A shows a first substrate 10 which comprises a substantiallysingle-crystal semiconductor material. The semiconductor material can bea silicon-based semiconductor or any other type of semiconductor, suchas, the III-V, II-IV, II-IV-V, etc. classes of semiconductors.

Examples of silicon-based materials which can be used for the firstsubstrate include silicon (Si), germanium-doped silicon (SiGe), andsilicon carbide (SiC) materials. Examples of other semiconductors whichcan be employed for the first substrate include Ge, GaAs, GaP, and InPmaterials.

The semiconductor of the first substrate will be in the form of asubstantially single-crystal material. The word “substantially” is usedin describing the first substrate to take account of the fact thatsemiconductor materials normally contain at least some internal orsurface defects either inherently or purposely added, such as latticedefects or a few grain boundaries. The word “substantially” alsoreflects the fact that certain dopants may distort or otherwise affectthe crystal structure of the bulk semiconductor.

As shown in FIG. 1A, the first substrate 10 has a first external surface11, a second external surface 12, and an internal zone 13 for separatingthe first substrate into a first part 14 and a second part 15, thethickness of the second part being Ds. As discussed above, D_(S) willtypically be in the range of 10 nanometers to 500 nanometers, withpreferred thicknesses being in the 10 nanometer to 150 nanometer range.If desired, the second part can be thicker than 500 nanometers, e.g., onthe order of a 1,000 nanometers or more. Similarly, the second part canbe thinner than 10 nanometers, although excessively thin semiconductorlayers will generally not provide sufficient material for the productionof semiconductor devices. Thinner semiconductor layers may be createdvia oxidation or other methods known in the art.

External surfaces 11 and 12 and separation zone 13 preferably areparallel to one another. However, to take account of the fact that theremay be some slight angle, e.g., up to 1-2 degrees, between one or moreof the surfaces and/or the zone, the surfaces and zone are describedherein as being “substantially parallel” which includes both thecompletely parallel and slightly angled cases. The phrase “substantiallyparallel” also includes the possibility that one or more of the surfacesor the zone may not be completely flat.

Separation zone 13 is formed using implantation/exfoliation techniquesof the type currently known to those skilled in the art or which may bedeveloped in the future. At present, the separation zone is preferablyformed using the hydrogen ion implantation techniques of the referencesdiscussed above, the relevant portions of which are hereby incorporatedby reference. Other currently-known techniques can also be used to formthe separation zone, such as, co-implantation of hydrogen and heliumions or hydrogen and boron ions. Whatever technique is chosen, the firstsubstrate needs to be separable into the first and second parts at theseparation zone during step (D). Thus, the separation zone needs torespond to the heat treatment/cooling process by becoming weaker so thatthe division of the first substrate into the first and second parts canoccur.

As also shown in FIG. 1A, the second substrate 20 comprises two externalsurfaces 21 and 22 which, like the surfaces and separation zone of thefirst substrate, are substantially parallel to one another. In order toensure that the SOI structure has uniform properties in, for example,the radial direction for a circular wafer, e.g., uniform bondingstrength at the interface between the first and second substrates, anydeviations from parallel of external surfaces 11, 12, 21, and 22 andseparation zone 13 are preferably kept to a minimum.

The second substrate comprises an oxide glass or an oxide glass-ceramic;although not required, the embodiments described herein include an oxideglass or glass-ceramic exhibiting a strain point of less than 1,000° C.As is conventional in the glass making art, the strain point is thetemperature at which the glass or glass-ceramic has a viscosity of10^(14.6) poise (10^(13.6) Pa·s). As between oxide glasses and oxideglass-ceramics, the glasses are presently preferred because they aretypically simpler to manufacture, thus making them more widely availableand less expensive.

As shown in FIG. 1A, the second substrate has a thickness D₂, which ispreferably in the range of 0.1 mm to 10 mm and most preferably in therange of 0.5 mm to 1 mm. For some applications of SOI structures,insulating layers having a thickness greater than or equal to 1 micronare desirable, e.g., to avoid parasitic capacitive effects which arisewhen standard SOI structures having a silicon/silicon dioxide/siliconconfiguration are operated at high frequencies. In the past, suchthicknesses have been difficult to achieve. In accordance with thepresent invention, an SOI structure having an insulating layer thickerthan 1 micron is readily achieved by simply using a second substratewhose thickness is greater than or equal to 1 micron. A preferred lowerlimit on the thickness of the second substrate is thus 1 micron.

In general terms, the second substrate needs to be thick enough tosupport the first substrate through the process steps of the invention,as well as subsequent processing performed on the SOI structure.Although there is no theoretical upper limit on the thickness of thesecond substrate, a thickness beyond that needed for the supportfunction or that desired for the ultimate SOI structure is generally notpreferred since the greater the thickness of the second substrate, thelower the electric field strength within the substrate during step (C)for the same applied voltage difference.

The oxide glass or oxide glass-ceramic is preferably silica-based. Thus,the mole percent of SiO₂ in the oxide glass or oxide glass-ceramic ispreferably greater than 30 mole % and most preferably greater than 40mole %. In the case of glass-ceramics, the crystalline phase can bemullite, cordierite, anorthite, spinel, or other crystalline phasesknown in the art for glass-ceramics. The glass phase of theglass-ceramic should be sufficient to allow movement of positive ionsaway from the interface between the first and second substrates duringstep (C).

Non-silica-based glasses and glass-ceramics can be used in the practiceof the invention, but are generally less preferred because of theirhigher cost and/or inferior performance characteristics. Similarly, forsome applications, e.g., for SOI structures employing semiconductormaterials that are not silicon-based, second substrates which are notoxide based, e.g., non-oxide glasses, may be desirable, but aregenerally not preferred because of their higher cost.

For certain applications, e.g., display applications, the glass orglass-ceramic is preferably transparent in the visible, near UV, and/orIR wavelength ranges, e.g., the glass or glass ceramic is preferablytransparent in the 350 nm to 2 micron wavelength range.

The glass or glass-ceramic of the second substrate can be produced fromconventional raw materials using a variety of techniques known in theglass making art.

The oxide glass or oxide glass-ceramic comprises at least some positiveions which during step (C), move within substrate 20 in the direction ofthe applied electric field, i.e., away from surface 21 and towardssurface 22. Alkali ions, e.g., Li⁺¹, Na⁺¹, and/or K⁺¹ ions, are suitablepositive ions for this purpose because they generally have highermobilities than other types of positive ions typically incorporated inoxide glasses and oxide glass-ceramics, e.g., alkaline-earth ions.However, oxide glasses and oxide glass-ceramics having positive ionsother than alkali ions, e.g., oxide glasses and oxide glass-ceramicshaving only alkaline-earth ions, can be used in the practice of theinvention.

The concentration of the alkali and alkaline-earth ions can vary over awide range, representative concentrations being between 0.1 and 40 wt. %on an oxide basis. Preferred alkali and alkaline-earth ionconcentrations are 0.1 to 10 wt. % on an oxide basis in the case ofalkali ions, and 0-25 wt. % on an oxide basis in the case ofalkaline-earth ions.

Although second substrates composed of a single glass or glass-ceramicare preferred, laminated structures can be used if desired. Whenlaminated structures are used, the layer of the laminate closest to thefirst substrate should have the properties discussed herein for a secondsubstrate composed of a single glass or glass-ceramic. Layers fartherfrom the first substrate preferably also have those properties, but mayhave relaxed properties because they do not directly interact with thefirst substrate. In the latter case, the second substrate is consideredto have ended when the properties specified for a second substrate areno longer satisfied.

Along these same lines, either or both of substrates 10 and 20 caninclude surface layers over part or all of their external surfaces,e.g., an oxide layer on the semiconductor. When present on surface 11 ofsubstrate 10 and/or surface 21 of substrate 20, such surface layersshould not have a composition and/or a thickness which will prevent theformation of a strong bond between the first and second substrates. Inparticular, an oxide layer on the semiconductor substrate having athickness greater than about 100 nanometers can lead to weak or nobonding with the glass or glass-ceramic substrate.

Although not wishing to be bound by any particular theory of operation,it is believed that an oxide layer having a greater thickness provides ahigh resistance to current flow and thus diminishes theelectrolysis-type reaction at the interface between the first and secondsubstrates which is believed to produce the desired strong bond.Accordingly, when an oxide layer is present on the bonding surface ofthe first substrate, it should function primarily as a passivationlayer, as opposed to an insulating layer. Likewise, any oxide layerformed on the bonding surface of the second substrate should notinterfere with current flow and thus will typically (and preferably)have a thickness of less than about 100 nanometers. When surface layersare present on the bonding surfaces of substrates 10 and/or 20, theyconstitute intermediate layers between the first and second substratesin the finished SOI structure.

For certain first substrate/second substrate combinations, pretreatmentof the bonding surface 11 of first substrate 10 to reduce its hydrogenconcentration has been found advantageous in achieving bonding of thesecond part 15 of the first substrate to second substrate 20. Inparticular, such a reduction in hydrogen concentration has been found tobe of particular importance when transferring silicon films from siliconwafers implanted with hydrogen ions to glass substrates containingalkaline-earth ions, such as, substrates made of CORNING INCORPORATEDGLASS COMPOSITION NO. 1737 or CORNING INCORPORATED GLASS COMPOSITION NO.EAGLE 2000™, which are used in, for example, the production of liquidcrystal displays. It is believed that a reduction in hydrogenconcentration will also be advantageous for glass and glass ceramicshaving high strain points, e.g., in the 850° C. to 900° C. range, whichare expected to be needed for RF applications in wireless and otherelectronics applications.

In particular, it has been found that after hydrogen ion implantation,the surface of an implanted silicon wafer has a high hydrogenconcentration, e.g., a high hydrogen ion concentration. The hydrogentermination at the Si surface inhibits the bonding process and thus ithas been found desirable to reduce the hydrogen concentration on theimplanted Si wafer surface by using a gentle oxidizing treatment inorder to obtain effective Si layer transfer to glass wafers of theforegoing types. Reduction in hydrogen concentration results in makingthe implanted silicon wafer more hydrophilic and allows the bonding totake place during the application of voltage and heat. The strong bondformed during the process allows uniform separation of the Si film fromthe mother wafer.

Quantitatively, it has been found that in the absence of a hydrogenreduction treatment, only about 10% of the glass wafer is covered with aSi film and even in the covered area, the Si film tends to benon-uniform. However, when the hydrogen concentration at the surface ofthe Si is reduced by an oxidizing treatment, a uniform Si film becomesattached to the glass wafer over its entire surface.

Various approaches can be used to reduce the hydrogen concentration onthe surface of an implanted wafer. Preferred approaches involve a mildoxidation treatment of the surface, such as, treatment of the wafer withan oxygen plasma, treatment with hydrogen peroxide, hydrogen peroxideand ammonia, hydrogen peroxide and ammonia followed by hydrogen peroxideand an acid, or combinations of these processes. Treatment with anoxygen plasma is the preferred approach, especially in a commercialsetting. Although not wishing to be bound by any particular theory ofoperation, it is believed that during these treatments, hydrogenterminated surface groups oxidize to hydroxyl groups, which in turnmakes the surface of the silicon wafer hydrophilic. The treatment ispreferably carried out at room temperature for the oxygen plasma and ata temperature between 25-100° C. for the ammonia+peroxide orammonia+peroxide followed by acid+peroxide treatments.

Although the foregoing discussion has been in terms of silicon wafers,it is believed that reductions in hydrogen concentration will beadvantageous for hydrogen-implanted semiconductor wafers composed ofsemiconductor materials other than silicon.

Turning to FIG. 1B, this figure illustrates step (B) of the process ofthe invention in which the first and second substrates 10 and 20 arebrought into contact at their bonding surfaces 11 and 21. In thepreferred embodiments of the invention, the first and second substratesare heated prior to step (B), e.g., are heated so that force-applyingsurfaces 12 and 22 are at T₁ and T₂, respectively. In this way,differential expansion between the first and second substrates isavoided during the bonding process of step (C). Alternatively, the firstand second substrates are not pre-heated prior to step (B), but areheated after bonding surfaces 11 and 21 have been brought into contactand before the beginning of step (C) and/or during the initial part ofstep (C) before substantial bonding has occurred. When pre-heating isperformed, the bonding surfaces can be separated by spacers which areremoved once the desired temperatures of the first and second substrateshave been reached.

The processing chamber, which is shown schematically at 40 in FIG. 1C,can have a variety of configurations. For experimental purposes, abonder of the type sold by SÜSS MICROTEC of Munich, Germany, can be usedas the processing chamber. The same equipment can be used for commercialapplications, although equipment capable of simultaneously processingmultiple first substrate/second substrate assemblies will generally bepreferred.

Because the invention uses low to moderate temperatures, pressures,electric field strengths, and vacuum levels, the requirements which theprocessing chamber needs to satisfy are not demanding, which is anotherimportant advantage of the invention, i.e., the invention can bepracticed with equipment which is both relatively inexpensive and widelyavailable or easily fabricated for custom applications.

FIG. 1C shows the central step of the process, i.e., step (C), where thefirst and second substrates are bonded to one another. Step (C) isperformed for a period of time sufficient for the first and secondsubstrates to bond to one another at the first and second bondingsurfaces. For example, step (C) can be performed for a period between 45and 90 minutes. Shorter periods of time are, of course, generallypreferred (e.g., times less than 30 minutes) and in a commercialsetting, it is expected that the time required to perform step (C) canbe reduced to a period of 5-15 minutes or less through the optimizationof substrate materials, processing temperatures, and applied voltages.

Step (C) is preferably performed under moderate vacuum conditions, i.e.,chamber 40 is evacuated while step (C) is performed. Preferably, thepressure in the chamber is less than or equal to 1 millibar, and mostpreferably, less than or equal to 10⁻³ millibars. Alternatively, step(C) can be performed in an inert atmosphere, such as, an atmosphere ofargon, helium, or the like.

As discussed above and shown in FIG. 1C, step (C) is performed withV₁>V₂ and preferably with T₁<T₂, where V₁ and T₁ are, respectively, thevoltage and temperature at force-applying surface 12, and V₂ and T₂ are,respectively, the voltage and temperature at force-applying surface 22.In the examples described below, the second substrate was located belowthe first substrate as shown in FIG. 1C, although the oppositeorientation can be used if desired. Also, vertical or other orientationsfor the substrates can be used, if desired.

V₁ and V₂ preferably satisfy the relationship:

100 volts/cm≦(V ₁ −V ₂)/D≦40 kilovolts/cm,

where D is the distance between the first and second force-applyingsurfaces during step (C). A preferred value for the (V₁−V₂)/D ratioranges between about 5-20 KV/cm.

T₁ and T₂ preferably satisfy the relationships:

T _(S)−350≦T ₁ ≦T _(S)+350; and

T _(S)−350≦T ₂ ≦T _(S)+350;

where T_(S) is the strain point of the oxide glass or oxideglass-ceramic and T_(S), T₁, and T₂ are in degrees centigrade. Asdiscussed above, T_(S) is less than 1000° C., can be less than 800° C.,and may also be less than about 700° C.

Typically, both T₁ and T₂ will be greater than or equal to 300° C. andless than or equal to 800° C., although higher or lower temperatures canbe used, if desired. Within this range, lower temperatures are generallypreferred, e.g., temperatures of around 450° C. for glasses like CORNINGINCORPORATED GLASS COMPOSITIONS NOS. 7070 and 7740 used in various ofthe examples presented below.

In addition to their role in achieving bonding of the first and secondsubstrates, as discussed above, T₁ and T₂ are chosen to providedifferential contraction of the first and second substrates upon coolingso that in the preferred embodiments of the invention, the secondsubstrate 20 seeks to contract to a greater extent than the firstsubstrate 10 to thereby weaken the first substrate at separation zone 13and produce an SOI structure where the semiconductor film is undercompression, as opposed to tension. Typically and preferably, T₂ will begreater than T₁, with T₁ and T₂ generally satisfying the relationship:

5° C.≦T ₂ −T ₁≦150° C.,

and preferably the relationship:

10° C.≦T ₂ −T ₁≦150° C.

Moreover, the coefficients of thermal expansion of the first and secondsubstrates and the chosen temperature differential will preferablysatisfy at least one and most preferably both of the followingrelationships:

CTE₁−20×10⁻⁷/° C.≦CTE₂≦CTE₁+20×10⁻⁷/° C.; and/or

(T ₂ −T ₁)·|CTE₂−CTE₁|≦30×10⁻⁵ , T ₂ >T ₁;

where CTE₁ is the 0° C. coefficient of thermal expansion of thesubstantially single-crystal semiconductor material and CTE₂ is the0-300° C. coefficient of thermal expansion of the oxide glass or oxideglass-ceramic. In applying these relationships, the 0-300° C. CTE of theoxide glass or oxide glass-ceramic (i.e., CTE₂) preferably satisfies therelationship:

5×10⁻⁷/° C.≦CTE₂≦75×10⁻⁷/° C.

As discussed above, during step (C), positive ions within the secondsubstrate (e.g., Li⁺¹, Na⁺¹, K⁺¹, Cs⁺¹, Mg⁺², Ca⁺², Sr⁺², and/or Ba⁺²ions (the alkali/alkaline-earth ions)) can move away from the interfacebetween the first and second substrates to form a depletion region,shown schematically in FIG. 2B by the reference number 23. The thicknessδ_(D) of this region can be defined in terms of reference concentrationsfor the positive ions.

Considering specifically the case of alkali/alkaline-earth ions, each ofthose ions which the oxide glass or oxide glass-ceramic contains willhave a reference concentration C_(i/Ref) at a reference surface 24which 1) is within the second substrate, 2) is substantially parallel tothe second bonding surface 21, and 3) is spaced from that surface by adistance D₂/2. The thickness δ_(D) of the positive ion depletion regioncan then be defined as the distance between the second bonding surface21 and a surface 23 a which 1) is within the second substrate, 2) issubstantially parallel to the second bonding surface, and 3) is thesurface farthest from the second bonding surface for which the followingrelationship is satisfied for at least one of the alkali/alkaline-earthions which the oxide glass or oxide glass-ceramic contains:

C_(i)(x)/C_(i/Ref)≦0.5, 0≦x≦δ_(D),

where

C_(i)(x) is the concentration of said at least one alkali/alkaline-earthion as a function of distance x from the second bonding surface, and

C_(i)(x) and C_(i/Ref) are in atomic percent.

Using this definition, δ_(D) will generally satisfy the relationship:

δ_(D)≧10 nanometers,

and will often satisfy the relationship:

δ_(D)≧1000 nanometers.

As discussed above, the movement of the positive ions within the secondsubstrate during step (C) also can produce one or more “pile-up” regionsfor one or more of the mobile positive ions contained in the oxide glassor oxide glass-ceramic. Such pile-up regions, when present, can have athickness of 500 nanometers or more and a peak positive ionconcentration C_(i/Peak) which satisfies the relationship:

C_(i/Peak)/C_(i/Ref)≧1.

where C_(i/Peak) and C_(i/Ref) are in atomic percent, and C_(i/Ref) isdefined as set forth above. In some cases, C_(i/Peak)/C_(i/Ref) will begreater than 2 (see, for example, the K⁺ pile-up region of FIG. 5Bbelow).

The one or more pile-up regions, when present, are located in thevicinity of x=δ_(D), i.e., they can overlap δ_(D) or can be just insideor just outside of δ_(D). In particular, the location x_(Peak) of thepeak of a pile-up region typically satisfies the relationship:

0.8·δ_(D) ≦x _(Peak)≦1.2·δ_(D),

and often satisfies the relationship:0.9·δ_(D) ≦x _(Peak)≦1.1·δ_(D),where x_(Peak) is the distance of the peak from the second bondingsurface and δ_(D) is as defined above.

In the case of oxide glasses containing alkaline-earth ions, it has beenfound that by reducing the processing temperature, time, and/or voltageemployed in step (C), some bonding of a silicon film to a glass wafercan be achieved with essentially no observable ion movement. However,this bonding is of the mechanical type, e.g., van der Waals typebonding, as opposed to chemical bonding, and the resulting bondstrengths are lower than those achieved with ion movement. Also, becauseit lacks a depletion region, the resulting structure can suffer from ionmovement into the silicon film during subsequent processing of the SOIstructure at elevated temperatures, which can impair the functioning ofelectronic devices formed on or in the silicon film. Accordingly, in thepreferred embodiments of the invention, the process is performed so thata depletion region is formed at the interface between the first andsecond substrates.

In addition to the depletion region and the one or more pileup regions,step (C) can also produce a hybrid region 16 of enhanced oxygenconcentration which begins at surface 11 and extends towards separationzone 13. The thickness δ_(H) of this region can be defined in terms of areference concentration for oxygen at a reference surface within thesubstantially single-crystal semiconductor material.

A suitable reference surface is, for example, surface 17 in FIG. 21which: 1) is within the second part of the first substrate, 2) issubstantially parallel to bonding surface 11, and 3) is separated fromthat surface by a distance D_(S)/2, where, as above, D_(S) is thethickness of the second part. Using this reference surface, thethickness δ_(H) of the hybrid region will typically satisfy therelationship:

δ_(H)≦200 nanometers,

where δ_(H) is the distance between bonding surface 11 and a surfacewhich 1) is within the second part of the first substrate, 2) issubstantially parallel to bonding surface 11, and 3) is the surfacefarthest from bonding surface 11 for which the following relationship issatisfied:

C_(O)(x)−C_(O/Ref)≧50 percent, 0≦x≦δ _(H),

where C_(O)(x) is the concentration of oxygen as a function of distancex from bonding surface 11, C_(O/Ref) is the concentration of oxygen atthe above reference surface, and C_(O)(x) and C_(O/Ref) are in atomicpercent.

Typically, δH, will be substantially smaller than 200 nanometers, e.g.,on the order of 50-100 nanometers. It should be noted that C_(O/Ref)will typically be zero, so that the above relationship will in mostcases reduce to:

C_(O)(x)≧50 percent 0≦x≦δ _(H).

In the case of a first substrate which is silicon based and a secondsubstrate which is a silica-based glass or glass-ceramic containing oneor more other oxides, e.g., such network formers as B₂O₃, Al₂O₃, and/orP₂O₅, the hybrid region can be characterized as a region which containssilicon oxide, e.g., silica (SiO₂), but does not contain the one or moreother oxides of the silica-based glass or glass-ceramic.

In summary, step (C) transforms the interface between the first andsecond substrates which is formed in step (B) by contacting bondingsurface 11 with bonding surface 21, into an interface region 30, whichpreferably comprises hybrid region 16 and depletion region 23, and mayalso preferably comprise one or more positive ion pile-up regions in thevicinity of the distal edge of the depletion region.

After step (C), the bonded first and second substrates are cooled, e.g.,to room temperature, and the first and second parts (14,15) of the firstsubstrate are separated from one another. Because of the weakening ofthe separation zone which occurs during cooling, this separation can beperformed without disturbing the bond between the second part and thesecond substrate or damaging the second part or the second substrate. Inmany cases, the separation involves merely moving the first and secondparts (14,15) of the first substrate 10 away from one another (e.g.,lifting the second part 14 upwards as shown in FIG. 1D), since duringthe cooling, those parts will have become completely free of oneanother. In some cases, a slight peeling action, like that used toremove household plastic wrap from a smooth object, is used at the endof the cooling to separate the two parts, but more than this is notneeded because of the differential contraction of the first and secondsubstrates and the resulting weakening of the separation zone.

As can be seen in FIG. 1D, the separation will typically result in partof separation zone 13 ending up associated with the first part of thefirst substrate and with part ending up associated with the second part(see 13a and 13b in FIG. 1D). Depending upon processing conditions andultimate end use, the external surfaces of the first and second partsproduced by this separation, i.e., the exfoliation surfaces, may beuseable as is or may require subsequent treatments, e.g., polishing,etching, doping, etc., prior to use. For example, prior to reuse as afirst substrate in another iteration of the overall process, theexfoliation surface of first part 14 may be subjected to conventionaltouch polishing to provide a sufficiently smooth surface for bonding toa new second substrate. Such polishing or other surface treatments mayalso be appropriate for the exfoliation surface of the second part 15prior to its use in the manufacture of a thin film transistor or otherelectronic device.

Although generally not preferred, one can conceive of integrating step(D) into step (C) by, for example, partially cooling the first andsecond substrates and then applying a separating force, e.g., twistingthe first and second substrates relative to one another, whilecontinuing to subject the substrates to an elevated temperature, anelectric field, and an applied pressure. Such separating can, forexample, be begun part way through step (C). In a commercial setting,such integration of step (D) with step (C) may be desirable to shortenthe overall process, especially if for a particular set of substratesand operating conditions, the bonding between the substrates becomesstrong enough for separation of the first and second parts to beperformed while additional step (C)-type processing is continued todevelop a depletion zone of a desired thickness.

As noted above, once the first and second parts are separated, theresulting SOI structure, i.e., the second part and its attached secondsubstrate, can be subjected to further processing as appropriate for theintended use of the structure. In particular, surface 13 b can, forexample, be treated to remove any roughness or other imperfectionsarising from the separation process. Similarly, first part 14 of thefirst substrate can be treated for subsequent use as, for example, a new(slightly thinner) first substrate.

FIGS. 3 and 4 illustrate a modification of the first substrate which canbe used if delamination (de-bonding) of the first and second substratesis a concern. Specifically, delamination can occur if there is a largedifference in the coefficients of thermal expansion of the semiconductormaterial and the oxide glass or oxide glass-ceramic. It can also occurwhen SOI structures having large surface areas are produced.

As shown in FIGS. 3 and 4, to address this problem, recesses 18 can beformed in the first substrate which begin at bonding surface 11 andpreferably extend into the first substrate to a distance greater thanthe depth of second part 15. These recesses form isolated regions 19which can expand and contract relatively independently of one another.In this way, high stresses do not develop as a result of differences inthe CTEs of the first and second substrates as those substrates, afterbonding, cool from the operating temperature of step (C), e.g., 450° C.for glasses like CORNING INCORPORATED GLASS COMPOSITIONS NOS. 7070 and7740 used in various of the examples presented below, to roomtemperature.

For many applications, the first substrate will be a silicon-basedsemiconductor material and the second substrate will be analkali-containing glass. For this case, the process of the invention canbe practiced as follows.

First, a glass containing a small percentage of alkali ions withexpansion relatively well matched to that of silicon is chosen. Asilicon wafer either without an oxide layer or with a thin oxide layer(see above) is implanted with hydrogen ions to generate subsurfaceflaws. The implanted wafer is then placed on the glass wafer with theimplanted surface next to the glass wafer surface with spacers inbetween.

These two wafers are then placed in a chamber. The wafer assembly isthen heated under a differential temperature gradient, with the glasswafer being heated to a higher temperature than the silicon wafer. Thetemperature difference between the wafers is at least 10° C., but may beas high as 100-150° C. This temperature differential is crucial for aglass having a CTE matched to that of silicon since it ensuresseparation of the Si film from the Si wafer due to thermal stresses.Without the application of the temperature gradient, the Si layerseparation cannot be performed without damage to the SOI structure. Oncethe temperature differential between the wafers is stabilized, thespacers are removed and mechanical pressure is applied to the two-waferassembly. The preferred pressure range is 1-50 psi. Application ofhigher pressures, e.g., pressures above 100 psi, typically causesbreakage of the glass wafer.

A voltage is applied across the wafer assembly with the Si wafer at thepositive electrode and the glass wafer at the negative electrode. Theapplication of the potential difference causes the alkali ions to moveaway from the Si/glass interface into the glass wafer. This accomplishestwo functions—an alkali free interface is created and the glass becomesvery reactive and bonds to the Si wafer strongly with the application ofheat at low temperatures.

After the assembly is held under these conditions for some time (e.g.,approximately 1 hr), the voltage is removed and the wafer assembly isallowed to cool to room temperature. The Si wafer and the glass waferare then separated, which may include some peeling if they have notalready become completely free, to obtain a glass wafer with a thin filmof Si, i.e., the desired glass SOI wafer.

As discussed above, in certain applications of the invention, multipleSOI structures can be formed on a single second layer. FIG. 11 shows astylized configuration for such a multiple first layer/single secondlayer assembly 50, where A₁, A₂, . . . , A₆ are representative firstlayer shapes and 51 are optional gaps between closely-spaced firstlayers. As discussed above, the portions of second layer 20 which arenot associated with a first layer can be used for a variety of purposes,including as supports for amorphous and/or polycrystalline semiconductormaterials.

FIG. 12 shows in schematic form the application of the process aspectsof the invention to the production of multiple first layer/single secondlayer assemblies. As shown in the left hand part of the figure,implanted semiconductor pieces 10, e.g., hydrogen-implanted siliconpieces, of the desired shape and size are assembled on a glass orglass-ceramic substrate 20 in an initial assembly step (hereinafterreferred to as “tiling,” which includes the case of a single firstsubstrate 10 which covers only part, rather than all, of a secondsubstrate 20). The resulting assembly of multiple semiconductor (e.g.,silicon) substrates and a single glass or glass ceramic substrate isthen subjected to the application of heat and electric potential to bondthe semiconductor to the glass or glass ceramic (see the middle part ofFIG. 12). All the semiconductor pieces bond to the glass or glassceramic although there may be no continuous electrical contact betweenthe individual pieces. On completion of the bonding cycle, thesemiconductor pieces and glass or glass ceramic with attachedsemiconductor film are separated to produce the desired SOI structure(see the right hand part of FIG. 12; this part of FIG. 12 also showsfilled gaps between the semiconductor films (see below)).

The advantages of using a tiling process include the ability to providelarge glass or glass ceramic substrates with substantially singlecrystal semiconductor films without a limitation on size. For displayapplications, the size of the glass substrate needed is often largerthan the 300 mm diameter of semiconductor wafers. Similarly,photovoltaic applications also require large area SOI structures.

Tiling also allows substantially single crystal semiconductor materialsto be placed on desired sites on glass or glass ceramic substrates. Thisability allows placement of high performance semiconductor films, e.g.,silicon films, in the areas of large substrates where drivers and memorycircuits may be placed and thus avoids having to cover the entiresubstrate with a semiconductor film, thus reducing cost.

When multiple semiconductor substrates are tiled on a single glass orglass-ceramic substrate, the distance between the semiconductor films ofthe finished SOI structure depends on the proximity of the semiconductorsubstrates during initial assembly. The proximity may be controlled byfinely machining the semiconductor pieces to precisely fit closetogether. FIG. 13A shows one way in which the edges of semiconductorwafers may be machined to minimize the gap between adjacent pieces.

FIG. 13B shows another way to perform the tiling operation in whichpieces of one or more semiconductor wafers 10 are assembled in a desiredpattern and then bonded to a conductive substrate 41 which acts as asupport structure. The bonding can be done by soldering, brazing, or useof a refractory conductive glue. The support structure may be a metalfoil or other conductive substrate which can withstand the processtemperature. The semiconductor pieces on the conductive substrate arethen implanted with, for example, hydrogen ions, and bonding to theglass or glass ceramic is carried out as described above. Afterseparation of the semiconductor films from the bodies of thesemiconductor pieces, the exposed exfoliation surfaces of thesemiconductor pieces on the conductive substrate can be polished toremove surface roughness, and implanted again, whereupon, the bondingprocess with another glass or glass ceramic substrate can be repeated.In this way, the semiconductor pieces do not need to be reassembled eachtime an SOI structure is produced. Tiling using a conductive support isparticularly useful when large area SOI structures are to be produced.

If desired, the small gaps between semiconductor pieces can be filledwith semiconductor material after the pieces have been assembled on theconductive substrate using CVD or other deposition processes. FIG. 14shows the resulting assembly, where the filled gaps are identified bythe reference number 52. Filling of all gaps removes any non-conductingregions from the semiconductor substrate. After the gaps have beenfilled, the ion implantation, bonding, and separation steps are carriedout to produce a continuous SOI structure having two or moresubstantially single crystal semiconductor regions separated by gapsfilled with amorphous semiconductor, e.g., amorphous silicon. The righthand portion of FIG. 12 shows such a continuous SOI structure obtainedby gap filling prior to bonding. If the deposition process is carriedout at high enough temperatures, the amorphous semiconductor materialmay crystallize to produce a substantially single crystal semiconductorfilm on the glass or glass ceramic substrate without any gaps. Again,these aspects of the invention are particularly useful when large areaSOI structures are to be produced.

Without intending to limit it in any manner, the present invention willbe more fully described by the following examples.

In Examples 5-8 and 13, elemental depth profiles obtained usingtime-of-flight secondary ion mass spectrometry (ToF-SIMS) are presented.As known in the art, ToF-SIMS is a surface analytical technique thatuses an ion beam to remove small numbers of atoms from the outermostatomic layer of a surface.

In broad outline, a short pulse of primary ions strikes the surface, andthe secondary ions produced in the sputtering process are extracted fromthe sample surface and into a time-of-flight mass spectrometer. Thesesecondary ions are dispersed in time according to their velocities(which are proportional to their mass/charge ratio i/z). Discretepackets of ions of differing mass are detected as a function of time atthe end of the flight tube. ToF-SIMS is capable of detecting ions over alarge mass range and can generate an image of the lateral distributionof these secondary ions at spatial resolutions of better than 0.15microns. Pulsed operation of the primary beam allows insulating surfacesto be completely neutralized between pulses using a low energy electronbeam.

ToF-SIMS was chosen to analyze the SOI structures produced by theinvention because the analysis could be performed without any chargebuild-up at the surface of insulators. Two ion beams were employed usingthe dual beam strategy—one for intermittent sputtering and another foranalyzing the newly created surface. Analysis was performed using aTRIFT II instrument manufactured by Physical Electronics, Inc., EdenPrairie, Minn. A low energy Cs beam was used to sputter insynchronization with a pulsed Ga beam for analysis. A small piece of thesample was cleaved to fit the ToF-SIMS sample holder (˜1 cm²). A 5 kV¹³³Cs⁺ beam was used for sputtering in conjunction with 15 kV, 600pA⁶⁹Ga⁺ beam for analysis. The Cs beam was rastered over 50 □m×50 □m areaof the sample; the Ga beam analyzed a 50 □m×50 □m window at the centerof the Cs-sputtered region.

For each of Examples 5-8 and 13, two types of plots are presented: onetype showing the variation of SIMS signal intensities and the other typeshowing an approximate quantification of these intensities to atomic %.The nominal composition of the base glass was used as an internalstandard in accordance with the “relative sensitivity factor” method.The ion signals towards the end of the profile were taken ascorresponding to the bulk glass. The ratio of ion intensities X⁺/Si⁺ wasused in these calculations, where X denotes a glass component and Sirepresents the matrix element. The sputter rate of the Cs beam wasdetermined by measuring the crater depth at the end of the experiment.Since this calibration assumes that the sputter is uniform across theanalyzed depth, the thickness of each layer could be somewhat different.Thus, as indicated in FIGS. 5B through 8B, the depths, as well as theatomic percent values, are only approximate. Although not explicitlyindicated on FIG. 10C, the depths and atomic percent values of thisfigure are also only approximate.

Each profile of Examples 5-8 and 13 is characterized by the followinggeneral sequence of layers.

(1) Native oxide on the surface of Si

(2) Silicon film

(3) Interfacial silica film (hybrid region)

(4) Alkali-depleted region (or alkaline-earth depleted region in thecase of Example 13)

(5) Alkali pile-up (or alkaline-earth pile-up region in the case ofExample 13)

(6) Base glass

In these SIMS depth profiles, higher Si⁺ signals are observed fromoxides such as silica and glass regions than from the elemental Si film.This enhancement of the SIMS signal is due to the presence of oxygen andis a well known example of “matrix effects” in SIMS quantification. Thiseffect is counter-intuitive because the elemental silicon film is 100 at% Si whereas the oxide is only 33 at % Si. The SIMS signal from theoxide is significantly larger by more than an order of magnitude. Thepresence of oxygen at the surface increases the work function therebyreducing the probability of neutralization of ions by electrons. We usedthis effect to delineate Si from silicon oxide, e.g., SiO₂, in theseprofiles.

In the quantified plots of FIGS. 5B through 5B, Si distribution is notincluded as it is the matrix element and was not quantified. Therefore,the intensity plots, i.e., FIGS. 5A through 8A, show a betterdelineation of the various concentration regimes across the analyzeddepth. The depletion of alkali leads to higher B⁺, Al⁺ and Si⁺ signalsfrom this region as reflected in the intensity distribution plots.However, for this alkali-depleted region, no attempt was made to correctfor the loss of alkali elements on the concentration of B, Al and Si.Similar considerations apply to the depleted region for thealkaline-earth case. Each interface is slightly broadened due tosputter-induced damage.

EXAMPLE 1

A 4-inch diameter phosphorus-doped silicon wafer (hereinafter referredto as the “silicon wafer”) having a thickness of 0.525 mm was implantedwith hydrogen ions at 69 KeV at a concentration of 6×10¹⁶ ions/cm² usingcommercially available, room temperature, ion implantation techniques.The phosphorus-doped silicon had a 0° C. CTE of 24×10⁻⁷/° C. and a 300°C. CTE of 38×10⁻⁷/° C. The wafer was obtained from SiliconSense Inc.,Nashua, N.H., and, as reported by the manufacturer, had a resistivity of1-10 ohm cm. Boron-doped silicon wafers were also used in various of theexamples with equivalent results to the phosphorus-doped wafers.

A 4-inch diameter glass wafer having a thickness of 1 mm was washed indetergent, rinsed in distilled water, soaked in 10% nitric acid for 1hour, and finally rinsed again in distilled water. It was then placed ina clean room hood and allowed to dry.

The glass wafer was made of CORNING INCORPORATED GLASS COMPOSITION NO.7070. On a weight percent basis, this glass comprises: 72 wt. % SiO₂, 27wt. % B₂O₃, 1.5 wt. % Li₂O, and 0.5 wt. % K₂O. It has a 0-300° C. CTE of32×10⁻⁷/° C. and a resistivity of 10^(11.2) Ω-cm at 250° C. and 10^(9.1)Ω-cm at 350° C.

The glass wafer was placed in a fixture for subsequent mounting on achuck connected to the “negative” support of a SÜSS MICROTEC bonder(Model SB6) with spacers placed on the top surface of the glass. Thespacers were brass and had a thickness of about 0.1 mm. The siliconwafer was placed on top of the spacers with the hydrogen-treated surfacetowards the glass. The fixtures clamps were then engaged over both thewafers.

The fixture was then placed in the bonder, vacuum was applied down to10⁻³ millibars, and the wafers were heated to 450° C. When the wafersreached the desired temperature, the spacers and the clamps were removedand ram pressure was applied to the wafers (10 psi) through the bonder'splungers. Thereafter, voltage was applied to the wafers through theplungers. Specifically, the difference in potential between the top ofthe Si wafer and the bottom of the glass wafer was 1000 volts. The(V₁−V₂)/D ratio was thus 6.55 KV/cm.

The wafers were held under these conditions for 1 hour. Finally thetemperature, pressure, and voltage were all turned off and the samplewas allowed to cool for a period of 2-3 hours.

It was found that the Si and glass wafers had bonded strongly. However,the desired separation of the glass wafer with a thin Si film from therest of the silicon wafer could not be achieved because both waferscracked during separation.

The experiment was repeated a number of times with the same result. Theglass and the silicon wafers were strongly bonded together but allattempts at separating the wafers resulted in cracking of both thewafers. Varying the cool down period was investigated, but with nosuccess.

EXAMPLE 2

The experiment of Example 1 was repeated except that the two wafers,glass and silicon, were held at 450° C. and 400° C. respectively. Thiswas done to take advantage of the thermal stress generated by thethermal differential to separate the thin silicon film from the siliconwafer after cooling. The differential heating was achieved by selectingthe temperatures of the force-applying plungers of the bonder. The 450°C. and 400° C. values represent the temperatures of the plungers andthus the temperatures at the bottom surface of the glass wafer and thetop surface of the silicon wafer, respectively, i.e., the temperaturesat surfaces 22 and 12 in FIG. 1C

In this case, the glass wafer with a 0.4-micron layer of silicon (an SOIconfiguration) and the remainder of the Si wafer were easily separated,and thus an SOI glass wafer was successfully obtained. This resultindicates that the thermal stress generated by a thermal differential isnecessary in generating a glass-based SOI structure. In particular, thethermal stress generated by cooling from a thermal differential isnecessary for a glass wafer having a CTE matched to that of silicon.

The SOI structures formed using the procedure of this example weresuitable for use in electronic, display and solar cell applications.

EXAMPLE 3

In this example, the Si wafer was first etched with a pattern ofcircular islands separated from one another. The islands were about 150microns in diameter. This patterned wafer was ion implanted and then theprocess of Example 2 was used to transfer a Si layer about 0.4 micronsthick onto the glass wafer. An SOI structure with an excellent bondbetween the Si layer and glass wafer was created in this case.

This island technique is expected to be of particular value when makinglarge wafers where a thermal expansion mismatch between the glass andthe Si is most likely to lead to delamination or other damage to the SOIstructure.

The pattern etched or otherwise formed in the silicon wafer may be anygeometric pattern which produces isolated islands of silicon. The sizeand the distance between the islands can be adjusted according to therequirements. If desired, a thin layer of silicon may be deposited viastandard techniques to establish connectivity between all or some of theislands. The same approach can be used with other types ofsemiconductors.

EXAMPLE 4

The experiment of Example 1 was repeated without the application ofvoltage across the wafers. In this case, the two wafers did not bond atall indicating that the application of the voltage across the wafers isan essential part of the process.

EXAMPLE 5

An SOI structure prepared in accordance with Example 2 was subjected toa ToF-SIMS analysis in accordance with the procedures discussed above.The results are shown in FIG. 5, where FIG. 5A plots SIMS signalintensity data versus depth and FIG. 5B shows the same data converted toatomic percent.

Depletion region 23 as well as hybrid region 16 are marked in FIG. 5,along with K⁺ pile-up region 25. Beginning at a depth of about 4microns, all of the curves have returned to their bulk glass values.

EXAMPLE 6

An SOI structure prepared in accordance with Example 2 was held at 500°C. for 1 hour under vacuum (10⁻³ millibars). A ToF-SIMS analysis wasthen preformed. The results are shown in FIG. 6.

A comparison of this figure with FIG. 5 shows that the depletion andhybrid regions, as well as the Si, B, K, and Li curves, aresubstantially unchanged by the reheating. This is an important resultsince it shows that the SOI structures of the invention will be stableduring further processing and ultimate use.

EXAMPLE 7

The experiment of Example 2 was repeated using a glass wafer made ofCORNING INCORPORATED GLASS COMPOSITION NO. 7740. On a weight percentbasis, this glass comprises: 81.3 wt. % SiO₂, 12.6 wt. % B₂O₃, 2.19 wt.% Al₂O₃, and 4.2 wt. % Na₂O, as well as trace amounts of Fe₂O₃ and K₂O.It has a 0-300° C. CTE of 32.6×10⁻⁷/° C. and a resistivity of 10^(8.5)Ω-cm at 250° C. and 10^(6.6) Ω-cm at 350° C.

The same processing conditions, including temperature, pressure, andvoltage, and the same wafer dimensions were used as in Example 2.

A ToF-SIMS analysis was performed as in Example 5 and the results areshown in FIG. 7. Again, a depletion region, a hybrid region (identifiedby the word “silica” in FIG. 7A and the phrase “interfacial silicalayer” in FIG. 7B), and a pile-up region can be seen in this figure.

EXAMPLE 8

An SOI structure prepared in accordance with Example 7 was held at 500°C. for 1 hour under vacuum (10⁻³ millibars). A ToF-SIMS analysis wasthen preformed. The results are shown in FIG. 8, where the samenomenclature is used as in FIG. 7.

A comparison of this figure with FIG. 7 shows that the depletion,hybrid, and pile-up regions, as well as the overall shape of theindividual concentration curves, are substantially unchanged by thereheating.

EXAMPLE 9

A convergent beam electron distribution function (edf) was obtained forthe semiconductor layer of an SOI structure prepared in accordance withExample 2. FIG. 9 shows the resulting transmission micrograph. As can beseen in this figure, the quality of the single crystal layer of Si isexcellent.

EXAMPLE 10

Experiments were performed to evaluate the feasibility of obtainingsingle crystal silicon films on glass and glass-ceramic wafers viathermal bonding without the use of an applied voltage or a temperaturedifferential.

One key issue in bonding a silicon wafer to a glass or glass-ceramicwafer is the well-known problem of contamination of the silicon withmobile ions from the glass or glass-ceramic. In the display industry,for example, the silicon film needed for electronic devices is depositedon a glass substrate after the substrate has been coated with barrierlayers of silica and silicon nitride to prevent migration of ions in theglass to the silicon film.

Thermal bonding of silicon directly to a glass or glass-ceramic waferwill cause migration of ions into the silicon which will adverselyaffect the silicon performance and is thus undesirable. In spite of thisissue, experiments were done to evaluate whether such a bonding processis feasible.

The glasses and glass ceramics used in these experiments had differentstrain point temperatures depending on their composition. Glasses usedin display applications are processed below their strain points to avoiddimensional changes, which are not acceptable. The thermal bondingexperiments were thus limited to the strain point of the particularcompositions evaluated. The following examples describe the experimentswhich were performed.

EXAMPLE 10A

The glass wafer in this case was made of CORNING INCORPORATED GLASSCOMPOSITION NO. 7070, which, as discussed above, is analkali-borosilicate glass. The strain point of this glass is 450° C.

A 100 mm diameter glass wafer 1 mm thick was polished to a surfaceroughness of 0.1 nm rms. A silicon wafer 100 microns thick was hydrogenion implanted at room temperature at a dosage of 7×10¹⁶ ions/cm² and animplantation energy of 100 KeV. Both the wafers were cleaned using astandard method for cleaning glass, namely, a detergent wash, adistilled-water rinse, a nitric acid treatment, and a finaldistilled-water rinse, and then brought into contact at roomtemperature.

The wafer assembly was placed between platinum plates and then thermallyheated to 450° C. and held at temperature for 1 hour. At the end of thehour, the furnace was cooled and the plates removed. No bonding wasobserved between the glass and the silicon wafer other than a lowstrength frictional bond. The wafers could be easily separated byinserting a razor blade at their interface. There was no trace of asilicon film on the glass. The experiment was repeated at 525° C. withan identical result, i.e., no silicon layer was observed on the glasswafer.

EXAMPLE 10B

The experiment of Example 10A was repeated but in this case the waferwas made of CORNING INCORPORATED GLASS COMPOSITION NO. 7740, which, asdiscussed above, is an alumino-borosilicate glass. The strain point ofthe 7740 composition is 540° C., and thus the bonding experiment wasdone at 540° C.

The result was identical to that of Example 10A, in that no transfer ofa silicon film to glass occurred.

EXAMPLE 10C

The experiment of Example 10A was repeated with an alkaline-earthalumino-borosilicate composition, namely, CORNING INCORPORATED GLASSCOMPOSITION NO. 1737, a glass used in the display industry. The strainpoint of this glass is 660° C. and thus the experiment was done at 660°C. Again, in this case, no transfer of a silicon film to the glass waferwas observed. The experiment was repeated using a pretreatment of thesurface of the silicon wafer to reduce the surface's hydrogenconcentration (see Example 11 below). Again, no transfer of a siliconfilm was observed.

EXAMPLE 10D

The experiment of Example 10A was repeated using a glass-ceramic waferwith an alkali and alkaline-earth aluminosilicate composition. Thematerial had a strain point of 810° C. and thus the experiment wasperformed at 810° C. Again, no transfer of a silicon film to theglass-ceramic wafer was observed. Pre-treatment of the silicon wafer toreduce the hydrogen concentration of its surface (see Example 11 below)did not change the result.

EXAMPLE 10E

The experiments of Examples 10A through 10D were repeated with siliconwafers upon which an oxide layer having a thickness of about 0.2 micronshad been grown prior to hydrogen ion implantation. As in Examples 10Athrough 10D, in each case, no transfer of a silicon film to the glass orglass-ceramic was observed.

Based on these experiments, it has been concluded that thermal bondingof a silicon film from a silicon wafer implanted with hydrogen ions to alass or glass-ceramic wafer does not occur for bonding temperatures upto the strain point of the glass or glass-ceramic. Higher bondingtemperatures are not acceptable because they would result in deformationof the glass or glass ceramic, e.g., sagging and/or compaction of thematerial. Moreover, as mentioned above, even if such a transfer wereachieved at some significantly higher temperature, the issue of ioncontamination of the silicon film would still not be resolved.

EXAMPLE 11

This example demonstrates the value of reducing the hydrogenconcentration at the surface of a semiconductor wafer which is to bebound to an alkaline-earth containing glass or glass-ceramic.

EXAMPLE 11A

A silicon wafer 100 mm in diameter and 100 microns thick was hydrogenion implanted at a dosage of 8×10¹⁶ ions/cm² and an implantation energyof 100 KeV. The wafer was then cleaned in distilled water. The contactangle measured on this wafer was 70 degrees indicating a veryhydrophobic surface.

A glass wafer composed of CORNING INCORPORATED GLASS COMPOSITION NO.1737 was washed with FISHER SCIENTIFIC CONTRAD 70 detergent in anultrasonic bath for 15 minutes followed by a distilled water wash for 15minutes in the ultrasonic bath. Thereafter, the wafer was washed in 10%nitric acid followed by another distilled water wash. The silicon waferwas not cleaned in the detergent to avoid contamination of the wafersurface. Both wafers were given a final cleaning in a spin washer dryerwith distilled water in a clean room.

The two wafers were then processed using the processing steps of Example2 and the SÜSS MICROTEC bonder of that example. Thus, the glass waferwas placed on the bonder's negative plunger and the silicon wafer wasplaced on the positive plunger and held away from the glass wafer withspacers. The two wafers were heated to 525° C. (silicon wafer) and 575°C. (glass wafer) and then brought into contact.

In particular, a potential of 1750 volts was applied at the center ofthe silicon wafer for a minute using the moveable center portion of thepositive plunger and then the applied potential was removed. The spacerswere then removed and a potential of 1750 volts was applied across thewafer surface. The voltage was applied for 20 minutes at the end ofwhich the voltage was brought to zero and the wafers were cooled to roomtemperature. As in Example 2, the bonding was performed under vacuumconditions.

At the end of the bonding process, the wafers could be separated easily.However, there was only a partial transfer of a Si film to the glasssubstrate. In particular, only about 10% of the glass was covered with aSi film and even in the covered area, the Si film was very non-uniform.The process thus had not given a good quality Si film, as was desired.

EXAMPLE 11B

The experiment of Example 11A was repeated except the silicon wafer wasnow treated in the following way.

First, the silicon wafer was placed in a cold solution of ammonia:hydrogen peroxide-water in the ratio of 1:2:7, and then the solution washeated to 70° C. gradually. After holding the wafer in the solution for15 minutes at temperature, the solution was cooled and the wafer waswashed in the ultrasonic bath with distilled water for 2.5 minutes. Atthis point the wafer showed a highly hydrophilic character with acontact angle of 10 degrees. The silicon wafer was then washed in ahydrochloric acid: hydrogen peroxide: water solution in a 1:2:8 ratiosimilar to the process for the ammonia solution. The treated siliconwafer was then washed in distilled water.

The silicon wafer was then bonded to a 1737 glass wafer washed as inExample 11A. The same process cycle as in Example 1A was used. After thebonding cycle, the glass wafer had an excellent, uniform Si filmattached over its entire surface.

This example shows that surface treatment of the silicon wafer afterimplantation is needed to carry out Si layer transfer to a glasscomposition containing alkaline-earth ions.

EXAMPLE 11C

In this example, the wafer was cleaned with an ammonia and hydrogenperoxide solution, but not with the acid solution used in Example 11B.

The experiment of Example 11A was repeated except that the silicon waferwas now treated in the following way.

First, the silicon wafer was placed in a cold solution of ammonia:hydrogen peroxide: water in the ratio of 1:2:7, and then the solutionwas heated to 70° C. gradually. After holding the wafer in solution for15 minutes at temperature the solution was cooled and the wafer waswashed in the ultrasonic bath with distilled water for 2.5 minutes. Atthis point the wafer showed a highly hydrophilic character with acontact angle of 10 degrees. The treated silicon wafer was then washedin distilled water and the bonding process was carried out with a 1737glass wafer washed as in Example 11A, using the same process cycle as inthat example. After the bonding cycle, the glass wafer had an excellent,uniform Si film attached over its entire surface.

EXAMPLE 11D

This example shows that oxygen plasma treatment, instead of an ammoniaand hydrogen peroxide treatment, may also be used to surface treat andthus control the hydrogen ion concentration of a semiconductor wafer.

In this example, a silicon wafer was treated in an oxygen plasma at roomtemperature for ten minutes. This treatment was followed by a distilledwater rinse. The wafer was then dried and subjected to the bondingprocess as in Example 11A. As in Examples 11B and 11C, an excellent Sifilm covering the entire glass wafer was obtained.

EXAMPLE 11E

Experiments were carried out to measure the surface hydrogen ionconcentration after the surface treatments of Examples 11A through 11D.The hydrogen concentrations at the surfaces of the silicon wafers weremeasured using a ToF-SIMS analysis. Normalized intensity values wereobtained by dividing the individual peak intensity by the total spectralintensity and multiplying by 10000. For the surface treatment protocolof Example 11A, a value of 414 was obtained (hereinafter referred to asthe “as-implanted” hydrogen concentration).

The results of these experiments are set forth in Table 2. As showntherein, the normalized signal intensity for the silicon wafer treatedin accordance with Example 11B was 102, which represents a 75% reductionin hydrogen ion concentration compared to the as-implanted wafer, whilefor the treatment of Example 11C, the signal intensity was 101 or 76%less than the as-implanted value. For the oxygen plasma treatment, thesignal intensity was 144 or a 65% reduction compared to the as-implantedwafer. An experiment was also performed in which an aqueous solution ofhydrogen peroxide without ammonia was used to treat the silicon wafer.The normalized signal intensity in this case was 307 or 26% of theas-implanted value.

These examples demonstrate that a reduced hydrogen concentration on thesurface of a semiconductor wafer leads to improved film formation duringbonding to substrates containing alkaline-earth ions.

EXAMPLE 12

This example demonstrates the high bond strengths achieved in accordancewith the invention for silicon films transferred to glass waferscomposed of CORNING INCORPORATED GLASS COMPOSITIONS NOS. 7070,7740,1737, and EAGLE 2000™. The composition of the 7070 and 7740 glasses areset forth above in Examples 1 and 7, respectively; the composition ofthe EAGLE 2000™ and 1737 glasses correspond to Example 14 and thecomparative example of U.S. Pat. No. 6,319,867, respectively.

The process steps of Example 2 were used to prepare the SOI structureswith the addition of a hydrogen reduction pretreatment of the siliconwafer for the 1737 and EAGLE 2000™ glasses which include alkaline-earthions. Specifically, the silicon wafers for these glasses were treated inaccordance with the procedures of Example 11B, although any of Examples11B through 11D or other treatment approaches for reducing surfacehydrogen could have been used. Bond energy values were determined usingthe indentation procedure discussed above and the calculation procedureof the above-referenced Marshall and Evans publication.

In particular, to determine bond strength, the sample was mounted withthe coating surface facing up so that the coating surface was assessableby the indenter probe. To provide a stable, rigid support, the samplewas bonded to a 1.25″ diameter aluminum sample stub supplied with theNano Indenter II using double-sided tape.

The adhesion characteristics of the sample were then assessed byproducing an array of indentations using a range of indentation loads.The array was selected to survey the coating response as a function ofload and was carried out in more than a single general location to checkfor variability in coating response across the sample. Using standardnanoindentation protocols, the positions of the indentations werepre-selected using microscopic observation to identify appropriateregions of the surface for testing. In particular, the positions wereselected to avoid regions of the surface which showed evidence ofcontamination or handling damage.

The testing was conducted using a diamond indenter with the Berkovichgeometry. The loading of the indenter was computer controlled using aprogrammed sequence of loading and unloading conditions. The sequence ofconditions used was as follows:

(1) approach segment

(2) load segment (constant strain rate of 0.1 sec⁻¹ to maximum load)

(3) unload segment (unload rate=90% of maximum loading rate)

(4) hold segment

(5) unload segment

The peak load in step 2 was typically varied to attain peak loads of 8.5mN, 37.6 mN, 165 mN, and 405 mN. For each region of interest on thesample, a minimum of 2 indentations were made for each load. Dependingon the observed response, additional testing at intermediate loads wasused to identify the critical load required to induce delamination or todetermine the delamination versus load response.

Following indentation, the test locations were inspected for evidence ofcoating delamination using low to high magnification Nomarski DIC(differential interference contrast) optical microscopy. When present,film delaminations around the indentation sight are typicallyidentifiable from buckling or other surface irregularities. Nomarski DICis particularly sensitive to this type of surface disturbance due to itshigh sensitivity to very small height changes (as small as a fewAngstroms). If delaminations had been present, their radii would havebeen measured and the model of the Marshall and Evans reference citedabove would have been used to determine a critical interfacial fractureenergy that represents a fundamental measure of the coating adhesion tothe substrate. In the absence of delamination, as occurred for the SOIstructures of the invention, a lower bound to the interfacial fractureenergy was estimated by assuming some minimum delamination region thatis slightly larger (1-2 microns) than the size of the indentationitself.

In determining interfacial fracture energy, it was assumed that therewas no residual coating stress in the samples tested (i.e., it was isassumed that the coating was stress free) and that the indentationstress field was thus the sole driving force for film delamination. Forthis case, the strain energy release rate was calculated using equation12 from the Marshall and Evans reference:

G=(1−v ²)tσ _(i) ²/2E  (eqn. 12 from Marshall and Evans)

-   -   where:

σ_(i) =V _(i) E/2πta ²(1−v)  (eqn. 10 from Marshall and Evans)

-   -   v=Poisson's ratio of the coating material    -   t=coating thickness    -   E=Young's modulus of the coating    -   a=radius of the debonded region    -   Vi=volume of the indentation (based on indent size and indenter        shape)

For the silicon-based films of the invention, the following values wereused for v and E: v=0.25; E=160 GPa 0.25. Vi for the Berkovich identeris an experimental variable and it is based on the identer used and thuswas measured and varies for each test/data point, while the thickness ofthe silicon-based film (coating) was about 0.4 microns in all cases.

Initial experiments were performed with 7070 glass processed at 450° C.with the silicon wafer at 400° C. The 7070 glass contains Li and K asthe mobile alkali ions. In the bond energy test, the glass wafer failedbefore the bond with the Si film could be broken. Based on this result,a minimum in bond energy was estimated at 15-20 J/m².

For comparison, literature data shows the bond energy for a Si—SiO₂thermal bond created at 450° C. is 1 J/m². Thus, at an equivalenttemperature, the SOI structures of the invention have a bond energy thatis 15-20× higher than the thermal process.

The experiment was repeated with EAGLE 2000™ glass with the glass waferheld at 575° C. and the silicon wafer at 525° C. This glass does notcontain any alkali ions, the mobile ions being alkaline-earth ions,specifically, Ca, Mg and Sr. The bond strength measurements were carriedout as before, and again it was found that only a lower bound on thebond energy could be obtained because the glass wafer failed before thebond between the silicon film and glass wafer broke. The lower bound wasagain 15-20 J/m² indicating that the inventive process produces veryhigh quality and high strength bonds at low temperatures.

These experiments, as well as corresponding experiments with the 7740and 1737 glasses, are summarized in Table 3. In each case, the bond wasvery strong and the glass wafer broke before any delamination of thefilm from the glass was observed. The bond energy measured at the pointof breakage is thus a lower bound on the bond energy.

EXAMPLE 13

An SOI structure was prepared using CORNING INCORPORATED GLASSCOMPOSITION NO. 1737 and the procedures described above.

A wafer of the 1737 glass (0.7 mm thick) was washed and cleaned asdescribed above. A boron-doped silicon wafer 500 microns thick washydrogen ion implanted at room temperature at a dosage of 7×10¹⁶ions/cm² and an implantation energy of 100 KeV. The wafer was thentreated to reduce the hydrogen ion concentration on the surface usingthe procedures of Example 11B, although any of Examples 11B through 11Dor other treatment approaches for reducing surface hydrogen could havebeen used.

The two wafers were then placed on a support, separated from each otherby spacers. The assembly was loaded into the bonder, vacuum pulled onthe system, and temperature increased (top silicon wafer was raised to525° C. and that of glass wafer to 575° C.). The wafers were thenbrought in contact and voltage applied. After a 15 minute application ofvoltage, the voltage was removed, and the wafers cooled down. The wafersseparated into two during cooling, i.e., into the glass wafer with athin silicon film bonded thereto and the silicon mother wafer minus thethin film.

A ToF-SIMS analysis in accordance with the procedures discussed abovewas performed on the SOI structure obtained in this way. The results areshown in FIG. 10, where FIGS. 10A and 10B plot SIMS signal intensitydata versus depth and FIG. 10C shows the data converted to atomicpercent.

The barium depletion and pile-up regions are specifically marked inFIGS. 10A and 10C, and the overall alkaline-earth (AE) depletion regionis marked in FIG. 10B. The hybrid (silica) region at the interfacebetween the silicon film and the glass substrate is also marked in FIGS.10A and 10B. Beginning at a depth of about 0.6 microns, all of thecurves have returned to their bulk glass values.

As these figures show, a barrier (depletion) layer is formed between thesilicon film and the glass which will allow the structure to be used inthe manufacture of electronic components without concern for ionmigration into the silicon film. To demonstrate the stability of thebarrier layer, an SOI structure prepared as described above was heattreated at 595° C. for 2 hours in vacuum and a TOF-SIMS analysis wasperformed on the heat treated structure. The analysis revealed thatthere was no movement of ions due to the heat treatment and the barrierlayer was permanent.

EXAMPLE 14

This example demonstrates how the formation of the depletion and pile-upregions depends on process conditions. CORNING INCORPORATED GLASSCOMPOSITION NO. EAGLE 2000™ was used for these experiments. The SOIstructures were formed using the procedures discussed above and includeda hydrogen reduction pretreatment of the silicon wafer.

The following process conditions were varied: (1) glass temperature, (2)applied voltage, and (3) bonding time. The silicon wafer was in allcases 50° C. cooler than the glass wafer. The parameters determined werethe thickness of the depletion region (barrier layer) and the identityof the ions which exhibited a clear pile-up region.

The results are shown in Table 4. As can be seen therein, both time andtemperature cutoffs appear to exist for the creation of a barrier layerand a pile-up region. Specifically, from this data it can be seen thatthe processing time needs to be above 2 minutes and the processingtemperature needs to be above 350° C. to obtain a barrier layer and apile-up region for this glass. Although voltages less than 800 voltswere not studied, it is believed that a cutoff will also exist for thisprocess variable. Similar cutoffs will exist for other glasses, althoughthe specific values are expected to be different from those found forthe EAGLE 2000™ glass.

EXAMPLE 15

This example illustrates the application of the invention to theproduction of thin film transistors (TFTs) for display applications,including organic light-emitting diode (OLED) displays and liquidcrystal displays (LCDs).

TFTs are used in displays to control the switching of individual pixelelements. For optimum display performance, and also to enableintegration of driver electronics, the TFT material should have fastcarrier mobility with high uniformity. Current state-of-the-artprocesses rely on TFTs fabricated from amorphous or polycrystallinesilicon (poly-Si) films. However the carrier mobility in devices madefrom these materials is one to five orders of magnitude lower than inbulk silicon, with a variability of up to ±30%. Bulk silicon representsthe ideal material for fabricating TFTs, but prior to the presentinvention, no practical process has been developed capable of producinga substantially single crystal silicon film on display glass substrates.

The prior art process for producing TFTs consists of a series of stepsbeginning with deposition of barrier layers of SiN_(x), and SiO₂followed by amorphous silicon deposition and dehydrogenation of thefilm. After dehydrogenation, the silicon film is crystallized via laseror thermal crystallization to obtain a poly-Si film with electronfield-effect mobility of around 100-350 cm²/V sec. From this film, theTFTs are built via a series of steps consisting of photolithography,followed by deposition of a gate oxide by PECVD, metal gate deposition,and more photolithography steps followed by a hydrogenation step toobtain good performance (by passivating defects between crystallineregions). See Applied Surface Science, 9602, 1-13 (2003)

FIG. 15 shows a typical TFT structure for use in an LCD constructed inaccordance with the prior art process. In this figure, 60 represents theoverall TFT, 61 represents the glass substrate, 62 represents metalcontacts, 63 represents a metal gate, 64 represents an SiO₂ gateinsulator, 65 represents a poly-Si layer, and 66 and 67 represent thetwo barrier layers (SiO₂ and SiN_(x), respectively) that need to bedeposited before deposition of silicon.

As discussed above, the present invention provides a practical methodfor obtaining substantially single crystal silicon on glass (SOG)structures. These structures are particularly well-suited for furtherprocessing to build TFTs. Since the SOG comprises a substantially singlecrystal silicon film, the process to build TFTs in accordance with theinvention is different from the prior art process. In particular, theprocess represents a simplification over current processes. Table 5 setsforth the various steps of the prior art process which can be eliminatedthrough the use of the present invention.

In particular, for the TFT process of the present invention, thedehydrogenation and crystallization steps are not required since thesemiconductor film is already in substantially single crystal form. Assuch, the film can have electron mobilities of over 600 cm²/V·sec. Therehydrogenation step required in the prior art process is also notrequired. These simplifications in the TFT manufacturing process provideboth capital cost and throughput advantages, as well as the inherentimprovement in device performance.

The resulting high performance TFTs produced in accordance with theinvention allow for miniaturization of electronics, more uniform displayperformance, integration of driver circuits on the glass substrate, andsubsequent significant power savings in the completed displays. Theresulting high performance displays can be used in a variety ofproducts, including, palm tops, cell phones, and the like. Furtherapplication for the SOG structures of the invention can be found insilicon-on-insulator (SOI) electronics.

EXAMPLE 16

This example illustrates tiling of multiple first layers 15 on a singlesecond layer 20 (see FIG. 11).

A boron doped silicon wafer (100 mm diameter) with resistivity of 1-10ohm-cm was hydrogen ion implanted at a dosage of 8×10¹⁶ ions/cm² and anenergy of 100 KeV. An alumino-borosilicate glass wafer (specifically, awafer composed of CORNING INCORPORATED GLASS COMPOSITION NO. 7740) of100 mm diameter was cleaned using a standard method for cleaning glass,namely, a detergent wash, a distilled-water rinse, a nitric acidtreatment, and a final distilled-water rinse. The silicon wafer wasscored into two pieces and broken. The pieces were then cleaned indistilled water, an ammonia and hydrogen peroxide solution, dried andthen assembled on the glass wafer manually to minimize the gap betweenthe two silicon pieces.

The resulting assembly was then placed in the SÜSS MICROTEC bonder. Thebonder was evacuated and the assembly was heated to 450° C. on the glasswafer end and 400° C. on the silicon wafer end. The glass wafer wasplaced on the negative electrode and the silicon on the positiveelectrode. After the desired temperatures were achieved, a 10 psipressure was applied to ensure good contact between the wafers. 750volts were then applied to the center of the wafer for one minute tostart the bonding process and then the voltage was removed. At thispoint, 500 volts were applied to the entire wafer and the assembly heldunder these conditions for 15 minutes. After 15 minutes, the electricpotential was removed, and the assembly was cooled to room temperature.

Both silicon pieces could be easily removed from the assembly leaving athin film of silicon on the entire glass wafer. The distance between thefilms from the two silicon pieces was about 10 microns.

The foregoing procedures were repeated except that the silicon wafer wasbroken into five pieces and then was assembled on the 100 mm glasswafer. All five pieces were found to have left a silicon film behind onthe glass wafer at the end of the process.

From the foregoing disclosure, including the foregoing examples, it canbe readily seen that the present invention provides new and improved SOIstructures and new and improved processes for producing such structures.FIG. 16 is a schematic flow chart showing a preferred embodiment of theinvention in which SOG structures, specifically, silicon-on-glassstructures, are produced using various of the features of the invention.Among other things, this figure illustrates hydrogen ion implantation,bonding through a combination of voltage and heat, separation throughcooling, and reuse of the silicon wafer to reduce raw material costs.

Although specific embodiments of the invention have been described andillustrated, it will be apparent to those skilled in the art thatmodifications and variations can be made without departing from theinvention's spirit and scope. The following claims are thus intended tocover the specific embodiments set forth herein as well as suchmodifications, variations, and equivalents.

TABLE 1 Number Element 10 first substrate (i.e., a substantially single-crystal semiconductor material) 11 first bonding surface/first face offirst layer 12 first force-applying surface 13 separation zone 13asub-portion of separation zone after step (D) 13b sub-portion ofseparation zone after step (D)/second face of first layer 14 first partof first substrate 15 second part of first substrate/first layer 16hybrid region of first substrate after step (C)/hybrid region of firstlayer 16a distal edge/thickness defining surface of hybrid region 17reference surface, e.g., a reference plane 18 optional recesses in firstsubstrate 19 optional isolated regions of first substrate 20 secondsubstrate (i.e., an oxide glass or an oxide glass-ceramic)/second layer21 second bonding surface/first face of second layer 22 second forceapplying surface/second face of second layer 23 depletion region ofsecond substrate after step (C)/depletion region of second layer 23adistal edge/thickness defining surface of depletion region 24 referencesurface, e.g., a reference plane 25 pile-up region of second substrateafter step (C)/pile-up region of second layer 30 interface regionbetween first and second substrates after step (C) 40 processing chamber41 conductive support 50 multiple first layer/single second layerassembly 51 gap between closely-spaced first layers 52 filled gapbetween adjacent first substrates 60 TFT 61 glass substrate 62 metalcontacts 63 metal gate 64 SiO₂ gate insulator 65 poly-Si layer 66 SiO₂barrier layer 67 SiN_(x) barrier layer

TABLE 2 Normalized H+ % reduction Example No. Treatment intensity wrtimplant 11A implanted 414 0 11B NH4OH + H2O2 102 75 & H2O2 + HCl 11CNH4OH + H2O2 101 76 11D O2 plasma 144 65

TABLE 3 Process Glass/Si Temperature Process Time Bond energy SystemVoltage (V) (° C.)¹ (Minutes) (J/m²) 7070/Si 1000 450 30 >15 7740/Si 500450 15 >15 1737/Si 1750 575 15 >15 EAGLE 1750 575 10 >15 2000 ™/Si¹Glass temperature; silicon wafer temperature was 50° C. cooler.

TABLE 4 Glass Barrier Layer Temperature Voltage Process Time Thickness(° C.) (V) (Minutes) (Microns) Pile-Up Ions 350 1750 20 No barrier layerNo pile-up 575 1200 20 0.08 Mg, Sr 575 800 20 0.046 Mg, Sr 575 1750 200.1 Mg, Sr 595 1750 10 0.08 Mg, Sr 575 1750 10 0.096 Mg, Sr 575 1750 50.064 Mg, Sr 575 1750 2 No barrier layer No pile-up 595 1750 2 Nobarrier layer No pile-up

TABLE 5 Prior Art Present Invention 1) Starting material - uncoateddisplay glass Starting material - silicon-on-glass 2) Deposition ofSiN_(x) barrier layers by PECVD Unnecessary 3) Deposition of SiO₂barrier layer by PECVD Unnecessary 4) Deposition of amorphous siliconlayer. Unnecessary 5) Heat treatment to dehydrogenate the Unnecessaryamorphous silicon layer 6) Laser (ELA) crystallization of silicon layerUnnecessary 7) Photolithography and dry etch of Si Photolithography anddry etch of Si 8) Deposition of gate oxide by PECVD Deposition of gateoxide by PECVD 9) Deposition of gate metal by sputtering Deposition ofgate metal by sputtering 10) Photolithography and dry etch of gatePhotolithography and dry etch of gate 11) Ion implantation and dopantactivation Ion implantation and dopant activation 12) Deposition ofencapsulation oxide by Deposition of encapsulation oxide by PECVD PECVD13) Photolithography and dry etch of contact Photolithography and dryetch of vias contact vias 14) Deposition of contact metal by sputteringDeposition of contact metal by sputtering 15) Photolithography and dryetch of contacts Photolithography and dry etch of contacts 16)Rehydrogenate poly-Si layer Unnecessary

1-65. (canceled)
 66. A semiconductor-on-insulator structure comprisingfirst and second layers which are attached to one another eitherdirectly or through one or more intermediate layers, wherein: (a) thefirst layer comprises a substantially single-crystal semiconductormaterial; (b) the second layer comprises an oxide glass or an oxideglass-ceramic; and (c) the bond strength between the first and secondlayers is at least 8 joules/meter².
 67. The semiconductor-on-insulatorstructure of claim 66 wherein the bond strength between the first andsecond layers is at least 10 joules/meter².
 68. Thesemiconductor-on-insulator structure of claim 66 wherein the bondstrength between the first and second layers is at least 15joules/meter².
 69. A semiconductor-on-insulator structure comprisingfirst and second layers which are attached to one another eitherdirectly or through one or more intermediate layers, wherein: (a) thefirst layer: (i) comprises a substantially single-crystal semiconductormaterial; (ii) has first and second substantially parallel facesseparated by a distance D_(S), the first face being closer to the secondlayer than the second face; (iii) has a reference surface which 1) iswithin the first layer, 2) is substantially parallel to the first face,and 3) is separated from that face by a distance D_(S)/2; and (iv) has aregion of enhanced oxygen concentration which begins at the first faceand extends towards the second face, said region having a thicknessδ_(H) which satisfies the relationship:δ_(H)≦200 nanometers, where δ_(H) is the distance between the first faceand a surface which 1) is within the first layer, 2) is substantiallyparallel to the first face, and 3) is the surface farthest from thefirst face for which the following relationship is satisfied:C_(O)(x)−C_(O/Ref)≧50 percent, 0≦x≦δ_(H), where: C_(O)(x) is theconcentration of oxygen as a function of distance x from the first face,C_(O/Ref) is the concentration of oxygen at the reference surface, andC_(O)(x) and C_(O/Ref) are in atomic percent; and (b) the second layercomprises an oxide glass or an oxide glass-ceramic.
 70. Asemiconductor-on-insulator structure comprising first and second layerswhich are attached to one another either directly or through one or moreintermediate layers, wherein: (a) the first layer comprises asubstantially single-crystal semiconductor material, said layer having asurface farthest from the second layer which is an exfoliation surface;and (b) the second layer: (i) has first and second substantiallyparallel faces separated by a distance D₂, the first face being closerto the first layer than the second face; (ii) has a reference surfacewhich 1) is within the second layer, 2) is substantially parallel to thefirst face, and 3) is separated from that face by a distance D₂/2; (iii)comprises an oxide glass or an oxide glass-ceramic which comprisespositive ions of one or more types, each type of positive ion having areference concentration C_(i/Ref) at the reference surface; and (iv) hasa region which begins at the first face and extends towards thereference surface in which the concentration of at least one type ofpositive ion is depleted relative to the reference concentrationC_(i/Ref) for that ion (the positive ion depletion region).
 71. Asemiconductor-on-insulator structure comprising first and second layerswhich are attached to one another either directly or through one or moreintermediate layers, wherein: (a) the first layer comprises asubstantially single-crystal semiconductor material, said layer having athickness of less than 10 microns; and (b) the second layer: (i) hasfirst and second substantially parallel faces separated by a distanceD₂, the first face being closer to the first layer than the second face;(ii) has a reference surface which 1) is within the second layer, 2) issubstantially parallel to the first face, and 3) is separated from thatface by a distance D₂/2; (iii) comprises an oxide glass or an oxideglass-ceramic which comprises positive ions of one or more types, eachtype of positive ion having a reference concentration C_(i/Ref) at thereference surface; and (iv) has a region which begins at the first faceand extends towards the reference surface in which the concentration ofat least one type of positive ion is depleted relative to the referenceconcentration C_(i/Ref) for that ion (the positive ion depletionregion).
 72. A semiconductor-on-insulator structure comprising first andsecond layers which are attached to one another either directly orthrough one or more intermediate layers, wherein: (a) the first layer(i) comprises a substantially single-crystal semiconductor material and(ii) has a maximum dimension greater than 10 centimeters; and (b) thesecond layer comprises an oxide glass or an oxide glass-ceramic whichcomprises positive ions of one or more types, wherein the sum of theconcentrations of lithium, sodium, and potassium ions in the oxide glassor oxide glass-ceramic on an oxide basis is less than 1.0 weightpercent.
 73. The semiconductor-on-insulator structure of claim 72wherein the sum of the concentrations of lithium, sodium, and potassiumions in the oxide glass or oxide glass-ceramic on an oxide basis is lessthan 0.1 weight percent.
 74. A semiconductor-on-insulator structurecomprising first and second layers which are attached to one anothereither directly or through one or more intermediate layers, wherein: (a)the first layer comprises a substantially single-crystal semiconductormaterial; and (b) the second layer: (i) has first and secondsubstantially parallel faces separated by a distance D₂, the first facebeing closer to the first layer than the second face; (ii) has areference surface which 1) is within the second layer, 2) issubstantially parallel to the first face, and 3) is separated from thatface by a distance D₂/2; (iii) comprises an oxide glass or an oxideglass-ceramic which comprises positive ions of one or more types, eachtype of positive ion having a reference concentration C_(i/Ref) at thereference surface; (iv) has a region which begins at the first face andextends towards the reference surface in which the concentration of atleast one type of positive ion is depleted relative to the referenceconcentration C_(i/Ref) for that ion (the positive ion depletionregion), said region having a distal edge; and (v) has a region in thevicinity of said distal edge in which the concentration of at least onetype of positive ion is enhanced relative to C_(i/Ref) for that ion (thepile-up region).
 75. The semiconductor-on-insulator structure of claim74 wherein the at least one type of positive ion has a peakconcentration C_(i/peak) in the pile-up region which satisfies therelationship:C_(i/Peak)/C_(i/Ref)≧1, where C_(i/Peak) and C_(i/Ref) are in atomicpercent.
 76. A semiconductor-on-insulator structure comprising first andsecond layers which are attached to one another either directly orthrough one or more intermediate layers with a bond strength of at least8 joules/meter², said first layer comprising a substantiallysingle-crystal semiconductor material and said second layer comprisingan oxide glass or an oxide glass-ceramic wherein at least a portion ofthe first layer proximal to the second layer comprises recesses whichdivide said portion into substantially isolated regions which can expandand contract relatively independently of one another.
 77. Thesemiconductor-on-insulator structure of claim 76 wherein the recessesextend through the entire thickness of the first layer.
 78. Thesemiconductor-on-insulator structure of claim 76 wherein the bondstrength between the first and second layers is at least 10joules/meter².
 79. The semiconductor-on-insulator structure of claim 76wherein the bond strength between the first and second layers is atleast 15 joules/meter².
 80. The semiconductor-on-insulator structure ofclaim 76 wherein the oxide glass or oxide glass-ceramic has a 0-300° C.coefficient of thermal expansion which is greater than the 0° C.coefficient of thermal expansion of the substantially single-crystalsemiconductor material.
 81. The semiconductor-on-insulator structure ofclaim 76 wherein the oxide glass or the oxide glass-ceramic of thesecond layer: (a) has a 0-300° C. coefficient of thermal expansion CTEand a 250° C. resistivity ρ which satisfy the relationships:5×10⁻⁷/° C.≦CTE≦75×10⁻⁷/° C., andρ≦10¹⁶ Ω-cm; (b) has a strain point T_(S) of less than 1,000° C.; and(c) comprises positive ions whose distribution within the oxide glass oroxide glass-ceramic can be altered by an electric field when thetemperature T of the oxide glass or oxide glass-ceramic satisfies therelationship:T _(S)−350≦T≦T _(S)+350, where T_(S) and T are in degrees centigrade.82. (canceled)
 83. The semiconductor-on-insulator structure of claim 76wherein the first layer: (i) has first and second substantially parallelfaces separated by a distance D_(S), the first face of the first layerbeing closer to the second layer than the second face of the firstlayer; (ii) has a first layer reference surface which 1) is within thefirst layer, 2) is substantially parallel to the first face of the firstlayer, and 3) is separated from that face by a distance D_(S)/2; and(iii) has a region of enhanced oxygen concentration which begins at thefirst face of the first layer and extends towards the second face of thefirst layer, said region having a thickness δ_(H) which satisfies therelationship:δ_(H)≦200 nanometers, where δ_(H) is the distance between the first faceof the first layer and a surface which 1) is within the first layer, 2)is substantially parallel to the first face of the first layer, and 3)is the surface farthest from the first face of the first layer for whichthe following relationship is satisfied:C_(O)(x)−C_(O/Ref)≧50 percent, 0≦x≦δ_(H), where: C_(O)(x) is theconcentration of oxygen as a function of distance x from the first faceof the first layer, C_(O/Ref) is the concentration of oxygen at thefirst layer reference surface, and C_(O)(x) and C_(O/Ref) are in atomicpercent.
 84. The semiconductor-on-insulator structure of claim 76wherein the first layer has a surface farthest from the second layerwhich is an exfoliation surface.
 85. The semiconductor-on-insulatorstructure of claim 76 wherein the first layer has a thickness of lessthan 10 microns.
 86. The semiconductor-on-insulator structure of claim76 wherein: (a) the first layer has a maximum dimension greater than 10centimeters; and (b) the second layer comprises an oxide glass or anoxide glass-ceramic which comprises positive ions of one or more types,wherein the sum of the concentrations of lithium, sodium, and potassiumions in the oxide glass or oxide glass-ceramic on an oxide basis is lessthan 1.0 weight percent.
 87. The semiconductor-on-insulator structure ofclaim 86 wherein the sum of the concentrations of lithium, sodium, andpotassium ions in the oxide glass or oxide glass-ceramic on an oxidebasis is less than 0.1 weight percent.
 88. Thesemiconductor-on-insulator structure of claim 76 wherein the secondlayer: (i) has first and second substantially parallel faces separatedby a distance D₂, the first face being closer to the first layer thanthe second face; (ii) has a reference surface which 1) is within thesecond layer, 2) is substantially parallel to the first face, and 3) isseparated from that face by a distance D₂/2; (iii) comprises an oxideglass or an oxide glass-ceramic which comprises positive ions of one ormore types, each type of positive ion having a reference concentrationC_(i/Ref) at the reference surface; (iv) has a region which begins atthe first face and extends towards the reference surface in which theconcentration of at least one type of positive ion is depleted relativeto the reference concentration C_(i/Ref) for that ion (the positive iondepletion region), said region having a distal edge; and (v) has aregion in the vicinity of said distal edge in which the concentration ofat least one type of positive ion is enhanced relative to C_(i/Ref) forthat ion (the pile-up region). 89.-91. (canceled)
 92. Thesemiconductor-on-insulator structure of claim 76 wherein the first layerhas a thickness D_(S) which satisfies the relationship:10 nanometers≦D_(S)≦500 nanometers.
 93. The semiconductor-on-insulatorstructure of claim 76 wherein the first and second layers are directlyattached to one another.
 94. The semiconductor-on-insulator structure ofclaim 76 wherein the second layer is transparent.
 95. Thesemiconductor-on-insulator structure of claim 76 wherein the secondlayer has a thickness D₂ which is greater than or equal to 1 micron. 96.The semiconductor-on-insulator structure of claim 76 further comprisingan amorphous or polycrystalline semiconductor material attached to thesecond layer. 97-112. (canceled)
 113. A silicon-on-insulator structurecomprising first and second layers which are directly attached to oneanother, said first layer comprising a substantially single-crystalsilicon material and said second layer comprising a glass or aglass-ceramic which comprises silica and one or more other oxides asnetwork formers, said first layer comprising a region which contacts thesecond layer and comprises silicon oxide but does not comprise the oneor more other oxides, said region having a thickness which is less thanor equal to 200 nanometers.
 114. The silicon-on-insulator structure ofclaim 113 wherein at least one of the one or more other oxides which arenetwork formers is selected from the group consisting of B₂O₃, Al₂O₃,and P₂O₅.
 115. The silicon-on-insulator structure of claim 113 whereinthe substantially single-crystal silicon material comprises silicon andgermanium.
 116. The silicon-on-insulator structure of claim 113 whereinthe substantially single-crystal silicon material comprises silicon andcarbon.
 117. The silicon-on-insulator structure of claim 113 wherein thesecond layer is transparent.
 118. The silicon-on-insulator structure ofclaim 113 wherein the second layer has a thickness D₂ which is greaterthan or equal to 1 micron.
 119. The silicon-on-insulator structure ofclaim 113 wherein the bond strength between the first and second layersis at least 8 joules/meter².
 120. The semiconductor-on-insulatorstructure of claim 113 wherein the first layer has a surface farthestfrom the second layer which is an exfoliation surface. 121-127.(canceled)
 128. A liquid crystal display comprising thesilicon-on-insulator structure of claim 113.